I always test my 1200 and 300 baud modems by using a SIP line, I use Sipgate
https://www.sipgateteam.co.uk, (no connection) (no pun intended) with a local number as its free. I know that VOIP/SIP isn't ideal for modems but I have found it to work quite well at these low speeds.
I am in the process of adding dial up (1200/75 only) to the Telstar videotex service (
https://glasstty.com/wiki/index.php/...ideotex_System but it has stalled due to family commitments, one day...