IP Multimedia Subsystem, how 4G advanced services are managed over data as 4G, LTE are “data only” so wifi calling, VoLTE, etc uses IMS and MNOs have typically told MVNOs that they have to have their own MVNO IMS if it wants MVNO VoLTE, MVNO WiFi Calling etc. We have an MVNO VoLTE in a box solution which can also be used for IoT (vending machines, lifts, etc need IoT Voice).