Speech to Text during a call?

I am just beginning to make RTT calls to a hearing impaired person who reads my text at their end, then speaks back to me. It would flow much more naturally for both parties if I could also speak, dictating my text. Since the other party cannot hear me, I am able to give commands for punctuation, etc. Is it possible to use speech-to-text dictation during a call? When a cellular or FaceTime Audio call is in progress, the dictation button is no longer active.