How to add ring back tone support in PSTN network for fix telephone owners to select a tone or music for peoples call her/his number. The network switching protocol is SS7.
The common approach is to include in the ACM the "Optional backward call indicators" (Q.763/3.37) and set the bit A to 1. This sets "in-band information or an appropriate pattern is now available", like used for announcements with no answer. It worked with lot of switches (at least on ITU-T world).