Trying To Use TracFone SIM in AT&T Phone

Okay so I have a Safelink SIM card which is essentially thru TracFone which broadcast thru T Mobile (same frequency as AT&T). I put my Safelink / TracFone SIM in my AT&T Samsung Galaxy S4 and I got a screen asking for a Network or a SIM unlock PIN. How do I get that and who do I contact. Like i said, I have a TracFone or Safelink SIM and the phonr is an AT&T S4.

Sounds like the phone is locked to AT&T, you would need to talk to them.

AT&T's unlocking page is AT&T Wireless Consumer Device Unlock Request. But I believe that the phone has to currently be on AT&T for them to unlock it. (TracFone uses AT&T as a carrier, so the easier way would be to ask Lifeline [or TracFone] for an AT&T SIM.)