03.873GPPStage 2TSUser-to-User Signalling (UUS)
8.1 Interworking with GSM PLMN/ISDN network supporting only a maximum User-user information element length of 35 octets
If interworking occurs with a network supporting only a maximum of User-user information element length of 35 octets, no notification shall be given to the calling user or called user sending the user information.
8.2 Interworking with non-ISDN network
In the case of interworking with non-ISDN network or with a non-ISDN called user, a progress indicator information element indicating #1 "call is not end-to-end ISDN; further progress information may be available in-band" or #2 "destination address is non-ISDN", respectively, is sent to the calling user as part of basic call.
This progress information shall serve as indication that the requested service cannot be guaranteed.