If you reboot your phone, does it still happen? That is most common with carrier registration issues, so if a reboot doesn't help, try removing the SIM card, cleaning it, and re-inserting it. If that doesn't help, call your carrier and see if they can 'reset' your registration on the network. If all else fails, you might need to replace your SIM card.
Also, you can try someone else's SIM card just to make sure the problem isn't with your device (unlikely, but never impossible).