Can I use my metro pcs Sim card in an att iPhone?

Do not tell me metro is only cdma and can't take Sim cards. Obviously, they do.

No only you can flash sprint and Verizon iphone not AT&T because metro is CDMA like Verizon and sprint and AT&T is gsm

Metro PCS is CDMA and if there's a SIM card in your phone most likely it is for the 4G LTE service. Sorry.