Technically Speaking there is no use of SS7 explicitly in VOIP but in general voip network works in conjunction with other PSTN network so SS7 work with VOIP network for signalling via Signalling Gateway which converts SS7 signaling to SIGTRAN which carry the signalling over IP network and terminates at MGC which connects to other MGC via SIP/H.323
In the pure IP network there is no role of SS7 or SIGTRAN, two MGCs are connected over SIP or H.323 using IP backbone.
