LG C1500

I'm a sales rep for Cingular and the only issue I've been having with this phone is it doesn't read certain SIM cards. It's a great little phone for people who aren't looking to do anything special and it's perfect for the elderly who just want it for emergencies. My only issue with this phone is that it doesn't read the Gemplus SIM cards we carry after about a week and about two hours of talk time... we've been forced to use a different brand of SIM card. Has anyone else experienced this problem, or did we just get a bad batch?