How long has this been going on? If this is a new occurence, it is probably a minor snag with the network. Give it a few minutes and it should work. If this isn't the case, try resetting the phone (soft reset, i.e. simply power it off and restart it). If that doesn't do it, try a hard reset, i.e. remove the battery. If none of these options works, it's probably best that you get a hold of your service provider's customer support.