Has it only happened once, or a multitude of times? I think we've all experienced this on the rare occasion, it should only be a concern if it happens on a regular basis. If you're that concerned you can turn airplane mode ON for a minute and then turn airplane mode OFF it will reacquire the network. You can also power down, remove the sim card, reinstall and try that.
Also make sure that you haven't accidentally turned on iOS's Do Not Disturb feature. You can tell if DND is active from an icon of a moon at the top of your device.
It happens randomly but the other day it was with some regularity. I will watch more closely and reboot and reinitialize SIM if it happens again.
Thanks for the input
No problem. If it was all in the same day you could have also just been experiencing a little lack of service for a short while, it was probably intermittent and went un-noticed until a call came through.
That is very possible. I checked the DND and it is ok but I may have accidentally had it on at the time.
Thanks very much to both of you for your immediate response.