Most of the “smart” payphones utilized either a Quadrum / “Sentinel” housing (shown in the drawing below - left side) or GTE 120B (mostly used by Frontier). They differed by the location of the coin return lever. Some “western” style housings, originally used with coin lines, were later converted to smart payphones using retro-fit boards.

Elcotel, Inc., based in Sarasota, Florida, was a major company providing public access telecommunication networks and services for both domestic and international wired and wireless communication. Their product line included smart payphones, software systems to manage networks of smart payphone terminals, electromechanical “dumb” payphones, and replacement components. They also offered services like equipment repair, customer training, and technical support.

Elcotel was the largest smart payphone supplier in the US with about 45% market share.

Gores Technology Group (GTG) acquired Elcotel in January 2002 and operated it as QuorTech Global Solutions. GTG was a leading provider of telephony and network system management services in 12 countries. The merger made them one of the leading providers of public communication products in North America, with operations in Virginia and sales and research in Sarasota, Florida.

The most common Elcotel model in use was the Series 5. Earlier models existed but might not be even compatible with new NANP (North American Numbering Plan).

Read more in the Elcotel section.

Protel was found in 1983 - about one year after the breakup of Bell - and became the worlds leading payphone manufacture with their Patented “Smart Line Powered Technology” meaning the phone was powered via the phone line, an industry first. Thru out the 1980’s Protel sold over 1,000,000 payphones in over 72 countries. As time passed Protel payphones incorporated both wired and wireless (cell) technologies while providing coin and credit card payment methods. Protel was the first company to provide remote monitoring of its payphones in the field via ExpressNet. This allowed both the server and the phone to report amount in the coin box, non-completed calls or inactivity reports, number of calls and their numbers, hardware issues with the phone and much more.

Protel made a wide range of boards starting with the 1000, 2000, 4000, Western 8000, 9000, 310, 7000, 7000c. The 7000c boards were used in Protel Ascension and Ascension Plus.
In general, Protel boards are considered the best in quality, features and reliability.

The latest phone made by Protel was Ascension, introduced in 2000 as their answer to Nortel Millennium^{[citation needed]}. It featured LCD or VFD display and a RJ11 jack so the user could connect his own laptop with a modem to dial the ISP. Phone never gained such a popularity as a Millennium though. Protel later renamed it to Elite^{[citation needed]}.

Read more in the Protel section.

Nortel manufactured several different models of payphones over the years. They were most commonly used in Canada, as well as the United States. Their First and the only “smart” phone was the iconic Millennium. Not a COCOT per se, as it was mostly used by big telcos. Read more in the Millennium section.

Ernest Communications, Inc. was incorporated in 1997, though its roots in telecom go back to 1984, when Joseph Ernest founded Ernest Telecom, a company devoted to the design and manufacture of printed circuitry for public communications equipment. Paul Masters joined Ernest Telecom in 1992, and over the course of five years held various sales leadership and senior management positions before becoming President in 1996. Coincidentally, The Telecommunications Act was signed into law in 1996, the first major overhaul to telecommunications law in more than a half century. Their COCOT phone was the ETX model. Board would fit into AE 120B housing. Manuals and software for Windows 95/98/NT is available, but unfortunately programming the rates is difficult or impossible. Keypad connector is DB15, different than Protel or Elcotel.

Related documents:
ETX payphone manual
Telelink quick start manual
Photos of inside + how to override the rates
How to connect a regular 16x2 LCD display
Telelink 2.61 - Windows 95/98/NT4.0 software to program the payphone, includes manuals
ROM dumps from 1987 - from PhilTel & danc256

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Very little we know about those phones. Difficult to program due to limited availability of the software. BCSETUP.EXE is for the programming, there's another piece of software for setting up the rates (StarCheck / AstraCheck). A dedicated modem is needed (but we can't tell if it can be replaced by a regular one, it looks like a standard 1200 bps modem from the era).

AT&T wasn't very popular in the COCOT business, but there were a few models, in particular: Private Payphone and Private Payphone Plus. Some of them were recently available for sale, often in a NOS condition (new). Private Payphone can be programmed entirely from the internal keypad, unfortunately the “Plus” model needs a dedicated software (CoinSoft and CoinRates) that is currently difficult to get. It technically works out of the box, but only with the area codes from back then (so essentially x0y and x1y, eg. call to area code 480 would not go thru). However, the Western type housing looks great and the main controller can be swapped with a Protel 8000 for example (to be confirmed) or a “dumb” D1D chassis. Those payphones were not very popular, mostly were (allegedly) part of a payphone scam back in a days. Below a quote from http://www.classicrotaryphones.com:
There was a scam going on by several companies (…) to own your own payphone route. Thousands of people fell for the scam. The company would claim if you paid them $5,000 a phone they would set up the route, install the phones and all you would have to do is pay commission and collect the route. Problem was they charged you 4 times the price of the phone, then gave you crummy locations like a Chinese restaurant or a out of the way gas station. (…) Finally after probably 10 years or more it stopped due to lawsuits. Since the money was gone already the scammers were gone also, the lawsuit turned to AT&T because they said the AT&T logo on the phone implied it was AT&T that was the company pushing the routes. Which was total bull crap. Well AT&T agreed to a meager settlement just to make it go away. That is why you now find AT&T payphone and Ernest phones in the box. That is also why AT&T removed the logo. (…) I used to buy these phones up from folks all the time that got screwed for pennies on the dollars for parts. I would listen to the sad stories of how they lost all their savings.

Warning: there is a design flaw, the cable in the AT&T handset always looses the isulation over time and come in contact with the grounded shield, burning out the power supply and possibly a mainboard. It has to be replaced or fixed.

Related documents:
AT&T Payphone Plus ad
AT&T Provate Payphone manual
AT&T Provate Payphone Plus manual

Williams, which was a pinball and arcade company, came up with a payphone model 5000 around year 1985. They were powered with a 24V AC transformer. Phone generated a fake dial tone until it actually went on line to dial the number. The only problem with the 5000 was the “answer detect” circuitry wasn't all that reliable yet with people experiencing delays before the payphone's microphone would turn on when a call was answered. Because of those answer detect problems, Williams came up with the 6000 series board which was a considerable improvement and actually worked quite well. Then, all of a sudden, Williams decided to shut down their payphone division after only two years of operation. The payphone division was located in Gurnee, Illinois.

Related documents:
Wiltelco 5000 manual

GTE made their own smart board in the 1980s. Keypad and handset connector is the same as GTE120B (and consequently Elcotel-type) - 17-pin D-sub. Programming requires either a dedicated software, or a LCD module. I don't know anyone who would be able to program it, also doubt if it's NANP compliant.