As per my knowledge, three concepts have been defined for VOIP services. Those are as followings:
1. CS fall back.
In case of SRVCC, when UE moves from LTE to UMTS, its VoLTE call will be moved to 3G gateway and it will be converted into CS call .In essence, a packet switched voice call will be converted into circuit switched call. Another way is to do CS fall back when UE moves to CS network and when its voice call is over it comes back.