For circuit switched voice, the protocol is SS7 aka N7 signalling.
For packetized voice, H.248, SIP, SIGTRAN, MEGACO, MGCP etc can be used
Historically, following protocols have been used in circuit switched networks:
R2
Decadic
MF Socotel
TUP
N5 or CCIT N5
In modern circuit switched voice networks, above are rarely used and have been replaced by N7
If you want more info on the signalling protocols, see link below: