Can I Use A Locked Phone On Straight Talk. Straight Talk will unlock phones of current and former Straight Talk customers without charge. Ad Get 100 on Any New Lines Added by 1031.

As in you cant use a CDMA phone Sprint Verizon on a GSM carrier ATT TMobile But if you want to swich from one CDMA carrier to the other or one GSM carrier to the other just contact Straight Talk. You need to give IMEI number. Straight Talk APN Settings Step by Step Guide.
To use this phone with Straight Talks Verizon tower Straight Talk needs to check that the phone is compatible.
Its possible your ATT sim will work in that phone but if it does need to be unlocked you would have to contact Straght Talk for that. Non-former customers may request unlocking but Straight Talk may charge a reasonable fee. Straight Talk will not do it automatically. Please follow the steps.

