This problem "could be" a carrier issue, in which case you'll need to call you carrier's tech support. - - But if you want to try something out, do this:
#1 ) Go to settings/general management/reset/reset network settings - - choose that option & confirm; does that fix it? - - If not, you might want to call your carrier - tech dept. Good luck!
Btw, after resetting network settings, you'll need to re-pair any bluetooth devices.
Be sure to click " ✓ Accept as Solution" when you find an answer that works for you. - Thanks!