Try going settings > general > about. Do you see an option listed for iccid? Iccid is your SIM card. If it's not there more than likely the phone can't see the sim so its be something to do with sim reader as opposed to antennas.
1) Press & hold Greeting Key. “Record greeting” will announce with a confirmation tone. 2) Start recording your message. You can record up to a 30 second greeting. 3) When you are finished recording your greeting, press the Greeting or Set key. You hear a confirmation tone followed by the greeting you just recorded. Hope this helps!