You can try a hard reset and see if that helps. You can also try removing and reinstalling your SIM card. Try those couple of ideas and post your results.
Slingdog, oh good lord I hope you haven't listened to that loquat person already. Jumping to hard reset first...smh...
I would go into settings>general>reset>reset network settings. If that does not work you may need to power off and take out the SIM and then put it back in and turn back on. And also if none of that works turn on airplane mode and then turn airplane mode off.
Hope this helps
You do realize that doing a hard reset doesn't do anything to the phone other than clears the memory and clears the cache (which can be extremely helpful). Why would recommending a hard reset be such a bad idea? You stand to lose no data, apps, contacts, etc?