I meant to reply sooner, but life got in the way.
This post seems to confirm that some (probably older) Minicom IV's (which were the low end of the Ultratec line) used MX blacks rather than Alps.
https://geekhack.org/index.php?topic=30816.msg584475#msg584475As to the cost, if we're talking a TTY-to-TTY call, the cost would be no different than a regular payphone call. The call would be placed like any other call. The only additional cost involved would be if it's a relay call (TTY-to-voice). In that case, in the old days, the TTY user would pick up the receiver, dial the Relay Service number (toll free) on the phone keypad, and then when the operator answers, type the number the user wishes to call on the TTY keyboard. Nowdays 711 is the standard number to reach the Relay Service in the US and Canada. When it comes to payphone, local relay calls are free. Long distance calls, however, require either a calling card or a collect call. The relay service is generally paid for by a small surcharge (TRS) on your phone bill. Sometimes it's passed on to the customer, and some companies do not. I can't find the line item on my bill, though I know I've seen it when I had a different carrier. Nowadays, the Deaf community rely more on text messaging, IP Relay, or even a video phone. Depending on codec of course, TTY does not always work with VOIP. In the US and Canada, at least, TTY's generally used Baudot code at 45.5 baud.
What surprises me most about the photo you posted is that it's a standard home model TTY. The payphone TTY's that I have seen have (without exception) been reinforced steel models with chiclet keys, to reduce vandalism. They are generally sealed in a motorized case mounted under the payphone that opens in response to TTY signals, and retracts when the receiver is replaced at the end of the call.
They look like this:
http://www.thehightechstore.com/payphonetdd.htmlIn the following video, you can see how the Relay service works, and you can also hear typing sounds (they sure sound like Cherry MX) as well as hear what the Baudot code sounds like as it's transmitted over the phone line.