Your memory matches mine. The DTMF ABCD tones were used as a way of doing extra control on the call, eg in the US military's old system (autovon) they could indicate the priority of a call.
Your memory matches mine. The DTMF ABCD tones were used as a way of doing extra control on the call, eg in the US military's old system (autovon) they could indicate the priority of a call.