Have found that if you are on v5.1.1, no matter the device or carrier, that there is an issue with it getting confused between wifi and your cell network. I've done all the other items in past suggested (turning off the Smart Network switching in wifi, disabling the VoLTE option, etc). I finally caught not able to accept a call, at a friend's place, and then went though each available option to turn off. Once WiFi was turned off, calls came through, and when re-enabled, they again came through but hours later again got confused, and turning off/on wifi then allowed them through again. So, you either have to downgrade from 5.1.1 or turn off wifi when you are expecting texts/calls. Beyond that, keep hammering your carrier to push back at the device mfg to get the 5.1.1 update resolved.