With the shutting down of the 3G network, many phones will have to rely on 2G to carry voice and SMS services.    However, since the arrival of 4G, networks have been deploying Voice over IMS (VoIMS). This means using an IMS core to use IP-based voice services: namely VoLTE and VoWiFi.    If your customers’ devices do not support VoLTE or 4G Calling, then your customers may not be able to make calls (including to emergency services). You should check:    Devices: your subscribers devices should at least be compatible with VoLTE (the iPhone 6 was the first VoLTE-enabled phone)  Settings: subscribers should check their devices to see what voice settings are selected. These should be 4G, VoLTE or automatic.  Provisioning: to access VoIMS, the SIM needs to be provisioned correctly. Check Auriga, your APIs and your offer to see if your subscribers have the right voice access.