Hello, Pix! TFO operation is determined in 03.53. It is used inband codec negotiation between local and remote trau right after call setup. The local trau sends TFO_REQ message by stealing one bit out of 16. “This allows to have the least possible degradation of the PCM, since the TFO_REQ message is sent even in cases where TFO will not be possible e.g. MS to PSTN call.”.
If there is no TFO_ACK after several TFO_REQ then negotiation is stopped. I.e. TFO_REQ is sent on user channel on initial phase of call.
Interworking equipment in MSC also generates tones on user channel in PSTN direction. It may be DTMF tones or V-series modems procedures (e.g. V.34 etc.) depending on equipment type.
If server in PSTN generates DTMF tones then they are delivered to MS on user channel all the way (there is no Start-Stop DTMF messages in Netork-MS direction) with distortion by GSM codec in trau. It is MS responsibility to recognize such distorted tones. In this case it is really a weird solution.