Switching iPhone 6 to prepaid plan

[Newtons Apple]

If you paid full price then it should be unlocked. If you have not then you need to unlock it through a third-party.

You asked was there a way to check if it's unlocked and that is to insert a GSM SIM or you could restore your phone via iTunes and if it's unkocked you'd get an unkock message.

If you bought it outright then it's unlocked. You can just swap any sin in you want. That was incredibly pertinent information you omitted until now lol.

How on earth you're oaying $500 for three lines is beyond me though.

If the OP purchased it outright from AT&T at the time of introduction, the phone will NOT be unlocked.

The $500 is not rediculous as it may be multiple months of non-payment. Three phone plans can run up a bill quickly.