Dear, Pix and Ivann!
Obviously these modems use DTMF tones. they are realized by means of Start DTMF, Stop DTMF messages on signalling channel (FACCH on RF-interface) (during conversation) from MS to MSC and inband dual tone multifrequency (DTMF) signalling from MSC to PSTN (any tones and speech recognition equipment). Tandem Free Operation also uses inband signalling for codec negotiation hence it is possible some collisions. But TFO must be not used in PSTN direction.
B.R.