The Trimline telephone was designed by Western Electric and the industrial design powerhouse, Henry Dreyfuss and Associates in the early 1960's. It was preceded by the innovations of the Western Electric 500 P/U in 1955 and the Princess Phone in 1956. The concept behind the Trimline was 2-fold.
The first half of the Trimline design was the adaptation of a prior innovation which was wildly successful: the implantation of a light into the base of the phone which would illuminate the telephone dial at night or in dark places. The 500 P/U was a desktop set and the 552 P/U a wall phone which utilized the existing 500 and 552 base plate assemblies and 2 line switch assembly and bus strip. Western Electric added a screw base lightbulb and "mushroom" shaped housing in which a screw down mini-lamp was installed which ran between 6 and 14 volts AC. A small external power supply was used to illuminate the dial. The dial also had a clear plastic fingerwheel. You could switch the lamp on at half-brightness and use it as a nightlight, or leave it in the off position and the lamp would illuminate at full brightness when lifting the receiver off the hook. The lamp shined down on the fingerwheel and dial plate.
The Princess Phone, initially designated the 701 and later 702 took the 500 P/U innovation and instead mounted the lamp under the dial plate using a clear dialplate assembly and clear plastic fingerwheel. The dial plate had a small plastic white ring and opaque shroud over it ontop of which fit the clear dial plate. When the lightbulb turned on underneath the dial, the dial lit up. Needless to say this innovation placed the lightbulb inside the phone base.
As of the early 1960's, Western Electric was looking for another type of illuminated telephone, one that put the dial in the palm of the user's hand.
Its first experiment was the Schmoo Phone. This was an innovation which placed the dial or touchtone pad with the receiver and transmitter, much as you would find on a lineman's set, except the dial was on the inside of the handle instead of behind the ear cap where the receiver capsule was.
The Schmoo was named after a cartoon character as the phone vaguely resembled the shape of the cartoon.
Nonetheless, Henry Dreyfuss' company came up with an odd shaped handset, which unlike the Schmoo had straightened sides and a natural ergonomic curve to the outer shell. This design allowed Western Electric to put a small lighbulb in the handset to illuminate the dial or touchtone pad when you lifted the receiver. This design impacted the telephone industry dramatically.
As anyone can tell you, virtually every wirless telephone, whether a cell phone or a portable phone connected to a landline, is comprised of a trimline style design. The dial is in the handset. This design was Similar to a Siemens style phone which had a dial on the side of the transmitter area, and also similar to the Ericophone in that the dial in that one was in the base. But unlike both the Siemens and Ericsson phones, this had the dial or touchtone pad right there underneath the user's ear.
According to AT&T, the "dial-to-user" concept employed in the Trimline was to allow the subscriber to wander about with all of the necessary controls right there in the handset. There is a flash button which allows the user to hang up and redial, and then there's the dial. No need to return to the base of the phone to dial a number or hang up and redial.
The phones were initially manufactured at the Western Electric plant in Indianapolis, Indiana following trial runs on its model production also made at that plant.
The Trimline was offered on a limited basis in 1965 and by the following year it was offered throughout the country.
There are several different varieties of the Trimline which combines different base sets and hand sets and internal networks. The phone was offered in roughly 14 separate colors. In its final production run, swirl blue, swirl red and swirl yellow. "Swirl" being the code word for "marbelized".
Now down to the gritty nitty. The handsets contained various networks but were generally designated into two ranges, Types 220 and 226 (dial service) and Type 2220 and 2226 (touchtone service). Some contain a lightbulb some contain an LED or light emitting diode. There were several base apparatuses which were designated AC and AD.
To confuse matters worse, there is an 495, 895, 295 and a few other "flex PC" networks that fit inside the handset, the last of which involved a one-piece rigid "deck" which came out of the shell in one piece. The earliest of the handsets utilized a complex flexible PC network with the dial being screwed to it and the transmitter and receiver parts fitting into encapsulated enclosures. The receiver is a variation of the HA1 and G1 receiver capsules whereas the transmitter capsule is a standard G1/G3 transmitter capsule. Later models removed the matching paper back behind the touchtone pad light pipe and replaced it with a thin, shiny aluminum plate with the keypad lighting through the buttons underneath.
Because of all of the variations, you have to concentrate on what it is you want.
Dial Service Phones: All rotary dial trimlines are not alike. However the oldest varieties have the wider handset cord running between the base and handset. These older cords were 5 conductor cords of which generally, with a few exceptions, only 4 conductors were necessary. A modular adapter was developed by Western Electric to convert the older cord styles to the modular spring cords, but if you find a phone with the conversion in it, that means the dial was likely converted from incandescent lightbulb to LED.
Touchtone Service Phones: Touchtone service phones could have the wider plug in them, but generally do not. Typically they are modular. There are a few exceptions. First of all, they came in 10 and 12 button touchtone pads. The 10 buttons are more desireable because they are the oldest variety and will have the wider plug for the handset cord. They are also lighted with a lightbulb rather than an LED. As far as I know all 10 button phones have the smaller round touchtone buttons.
The 12 button phones came in a little later and came in little round buttons and large square buttons. Typically if you find a round button set whether 10 or 12 buttons, then that means you will almost certainly have a lightbulb lighting the dial.
The square button phones were, as far as I know, exclusively made as modular and did not have the wide plug and adapter in them, with the exception of some cross-overs and conversion phones.
Color Sets: There were 14 colors if not more in which the Trimline was offered. Many of the earlier models were made from poured, pressure-injected colored thermoplastics. However, as production went along, it was not unusual for Western Electric to pick up older sets and prime and dip them into new colors. So you might get a moss green handset that has the outer handset shell made of colored plastic and the inside part with the transmitter and receiver having been primed and dipped in a matching color. Since these phones were made for rental rather than ownership, the phone company didn't care if it chipped or cracked. They would replace the phone since they owned it.
Red phones were typically dye-painted from white, ivory, light beige or beige phones. Black phones were typically black in color, same with pink, forest green and aqua blue in most cases. However, be advised that you can always tell if one has been painted by the phone company or painted by someone with a $4 can of spray paint. The dead giveaway is orange peel. If there is orange peel on the phone anywhere, it means it was spray-painted. If it appears to have a smooth, shiny or lightly textured surface and even the appearance without any texture with a glass-like appearance, it was primed and then dipped. However that is still no guaranty that it was not recently "colorized".
In all fairness, after the breakup of AT&T in 1983-84, AT&T opened "phone stores" where they sold "reconditioned" telephone sets. Some of these might have a very slight almost imperceptible orange peel to them or appear to have micro-cracks from the cheaper paints that they used at that time. AT&T was anxious to get rid of all of its rental phones now that they were having trouble leasing phone equipment to an American public that could hook up as many sets to their homes as they wanted.
Ideally when looking for a real colored phone rather than a painted one, beware of buzz-words like "overcoated" or "recoated" or "factory painted". If it looks too new, its probably like that diamond ring for a dime. Its probably not going to be "factory repainted or overcoated" as Western Electric was the factory but not anymore and has not been for decades. If it was factory painted and there is some wear around the earpiece or down below the transmitter, you would see a tinge of white primer peeking through. "Overcoats" and so-called "Factory Repainted" jobs usually have no such primers underneath. Also, if it was dipped you would not be able to detect the original color when you took the phone apart and looked inside. All parts of the plastic would be dye-painted and soaked in through the polymer coating of the thermoplastic.
If there is fading in the color of the handset and base and the color is uniform all around both or parts thereof, then you likely have an unpainted and non-dye-painted set.
Cracks: The Trimline has several infirmities which lend it to cracking. Typically the base shell is thick and will not easily crack. However the handsets are a different story. The plastic is thin and gets brittle over the years. They tend to crack around the outer handset shell seam where you grip the phone around the edge of the earpiece or the transmitter. Also, they tend to crack around where the modular and large plug cords go in both in the early years and in the later modular-only versions. The handset plastics do not age well I'm afraid. Look for cracks all around the handset first. If its cracked then you are in for heartbreak unless its a rare color.
Lighting the Trimline: The phones with LED's in them are line-powered and do not need any external power source whatsoever. If it has the big square buttons for touchtone service, then it will likely have an LED underneath the transluscent dial pad.
If its a rotary phone, its easy to tell if an LED is present. Just rotate the dial and look between the 1 and 0 after moving the floating finger-stop. If it says, "LED" in small capital letters, then its line powered illumination.
If your Trimline has little round buttons or if its a dial phone and it does not have the LED between the 1 and 0 under the floating finger stop, then it likely has a lightbulb in it.
Check your line out. If you have the fixed hard-wired type line cord going to the wall, or a four prong jack on the end, the yellow and black wires in a four conductor cord will need to be hooked up to a power supply that provides 6 to 17 volts at 250 milliamperes per hour. The minimum voltage is 6 volts. A Western Electric 2012A all the way through 2012D (modular) power supply will do, or in the absence of that, I suggest use a cell phone charger, Class 2 AC Adapter that puts out 7.5 volts at 850mah and you should do fine. If you have a five conductor cord, then it should hook up to the white and black leads on the cord with yellow parking on with green inside the base or in the four prong plug or both. So you don't want to use yellow if there is a white conductor present.
In modular phones that have the lightbulb in the handset, its your outer conductors (yellow and black) to which to supply that 7 to 17 volts power. Please note! Do not hook up that power supply to the yellow and black leads on your phone line as there may be line power for a 2nd telephone line in your home or office. You should carefully make sure that the power going to that second pair (black and yellow) is isolated. When using modular jacks use a Line 1 and Line 2 splitter available at a local phone store or Radio Shack. You can power Yellow and Black separately that way.
LED vs. LIGHTBULB: The lightbulb is much, much brighter than the LED. There are two types of bulb mounts that fit into the Trimline. One mount fits the bulb sideways and the other pushes the bulb down. Either version sits next to the plastic clear dial plate which consists of the "light pipe". This uses light diffusion to illuminate the dial plate much as the princess phone bulbs do.
Later Versions: AT&T introduced the Model 220 trimline which has a memory button and automatic redial. It had many copy-cats. If it has a redial or memory button or anything other than 12 buttons or 10 buttons for the touchtone pad, or a dial in it, then its probably the later model. The later models had hollow bases and all the guts were in the handset. The base was nothing more than a cradle to hold the handset. The real deal has a ringer in it. True Trimlines have the ringer assembly and hookswitch in the base.
Copycat Trimlines: ITT and Stromberg Carlson made Trimline type phones, as did Automatic Electric/GTE. The ITT and Stromberg models look identical to the Western Electric versions, but there are distinctions. The parts are not easily interchangeable as there are slight variations and more importantly, the dial phones in the ITT's do not have the solid plastic "Western Electric" dial center in them that match the color of the phone. Instead they have a clear plastic center piece with white paper underneath it. The Strombergs do have solid plastics in the centers, but are unmarked generally or marked Stromberg Carlson. And also, some Strombergs have the clear plastic piece in the middle.
Summary: Decide which kind of phone you want and ask a lot of questions. If you find one that says "Factory Painted" or "Factory Overcoated" or whatever, ask which factory did it. Ask for documentation. If not assume someone just painted it recently. Look carefully at the plastic to make sure its not painted or if it was dye-painted then look to see if there is any natural wear and any white primer peeking through. As I said, it was not unusual for the Bell System to colorize their phones to meet custom orders. And AT&T did their own version of colorization in the 1980's on their refurbished stock.
Finally, examine the handsets around the edges. If there are cracks consider your purchase. Some colors are rare and hard to find. Teal Blue (unpainted), all the red colors in unpainted (including Rust and Cinnabar), and 10 button touchtones may be worth the purchase even if there are small cracks. I have one from 1965 that has a little crack. To me the phone is irreplaceable because it is date-stamped and matching all the way around and dates to the first year it was out). Also remember, the phone is what you make of it. If its an everyday use phone and you don't care, then buy cheap and painted and it will still work just fine. Western Electric made their phones to stand tall, and they still do very much so. Good luck.