Cell phone powers itself down.

[Joe Bob]

I have an AT&T cell phone with a SIM card. Problem occurs with multiple phones, only constant is the SIM card.

Would a defective card cause them to power down at it's whim? Specifically just after sending a text......other times just sitting on a desk with no activity. Battery is fine.....

[Aragorn]

How to Determine if a Cell Phone SIM Card is Damaged | eHow

I would take the phone to the nearest retailer and have the sim card checked.