i am not sure about ur case but everyone reporting here having issue is using maxis and i dont really think that maxis are selling faulty iphone 5 since day 1...i am a maxis user and having this issue as well on my iphone 5 and i not yet change my sim card from maxis center...but i manage to avoid restarting my phone but not completely by setting my carrier to manual instead of auto...select manually for my maxis carrier and once u encounter this issue, aeroplane mode should be helpful but not 100% as usual but at least avoid restarting phone 80% of the time. If this fail then restarting your phone would be the best choice...