My phone shows it has service, but says Emergency Calls Only.

This has always confused me. I supposedly can make an emergency call (to 911 I would guess) but I can’t make a call to other numbers. Can someone share their knowledge about this perplexing little situation?

EDIT: I guess I wasn’t clear enough, this doesn’t happen all the time, just when I am in remote locations and wouldn’t expect to have any service. When I’m in town I can make and receive calls just fine.