[kallin]

Sometimes my phone shows SIM Card Error, so no incoming calls come through.

Taking the SIM card out and inserting it back in (rebooting the phone in the process) tends to help, but occasionally I'll need to do this 3 or 4 times before the error goes away.

What causes this? Is there an issue with the SIM card, the phone, or the network?

[Trimix]

Either your SIM card is loose (not properly latched down), or it might be going. Your cell provider should usually provide a replacement.

[kallin]

I'm with AT&T. Would they be able to provide a replacement at the store, or should I call them instead?

[Dubdub]

The store should have one.

[unclefester]

Yeah chances are your sim card is just shot. My brother and I had both of our sim cards go bad at about the same time, which was about 2 years after we got them. Your service provider should be able to just get you a new card.