My random tech problems and chicken scratching for notes on what I did to solve them.
AT&T vs T-mobile sim card in straight talk phone
difference between att and tmobile sim card in a straghttalk phone or service:
look at the SIM card numbers. If it starts with 890126 its T-mobile. 890141 for AT&T. OR check bottom of packaging of the phone. Look for the white label. Look on the right for an "A" or "T" (AT&T or T-mobile)