I still use an UltraTec 1140 TTY-VCO. I don't "hear anything on the phone" when I have it VCO mode- just the Relay operator keying on the screen what the other party is saying. In Canada this is 711.
Aside: had the phone since January 1996.
further aside: Notwithstanding I have a Cochlear Implant-it doesn't pick up phone coversation. According to Sunnybrook/Toronto Cochlear Implant section- one out of two can't use their Implants on regular phones. I am not unduly concerned.