Yeah - I've had this issue sometimes. It seems to be a SIM issue. It's happened on nearly every phone I've owned.
What seems to happen is that the SIM disconnects from the network, but doesn't indicate this to the phone. If you call the phone it goes to voicemail. If someone sends you a text, you don't get it. If you try and send a text it just sits there doing nothing. When you restart the phone (or go to airplane mode and back to normal) the SIM reconnects to the network and everything goes through.
