Four mode of operations are defined as following :
- PS mode 1 of operation: the UE registers only to EPS services, and UE's usage setting is "voice centric";
- PS mode 2 of operation: the UE registers only to EPS services, and UE's usage setting is "data centric";
- CS/PS mode 1 of operation: the UE registers to both EPS and non-EPS services, and UE's usage setting is "voice centric"; and
- CS/PS mode 2 of operation: the UE registers to both EPS and non-EPS services, and UE's usage setting is "data centric".
As per your question statement, I think CS/PS mode 2 of operation is applicable since in this mode "IMS based voice service" has higher preference than "CS based voice".