I keep getting a message that no sim card is installed on my Iphone 4. When it happens I lose my at&t service. I can do a hard reset and everything is fine again for awhile, but the message will come back again and I just rinse and repeat with a reset. It first happened three days after I installed ios 4.3 and It took about a week for the message to reappear. Now it's worse to the point that I have to do a reset at least twice a day. I removed and reinstalled the sim card and that didn't help. My question is where do I go for help? Do I contact AT&T maybe to replace the sim card? Or, do I contact Apple for an issue with the Iphone? It's just weird that it happened after the install of 4.3.