For the last few weeks, I have been told by friends and family that they are attempting to call me but phone calls are not going through. On at least 3 instances, I have been on my phone (using an app) and have received imessages that someone has tried to call me and it keeps going straight to voicemail. I did troubleshooting with apple care and there are no issues with the device itself, per apple care, it must be a carrier issue. Any way to resolve this?