Bibbo
Did you maybe refer to the Standards GSM08_series?
e.g.
GSM 08.62: ” Digital cellular telecommunications system; Inband Tandem Free Operation (TFO) of Speech Codecs
See GSM08.62 Cl4 Principles of Tandem Free Operation
Also, have a look at GSM08.60 and see erlang com/forum/erlang/thread.htx?thread=5987
BR