Upgrading to iOS 7 & Carrier Lock

[TiMacLover]

I have a refurbished iPhone 5 I received recently from Apple. My previous iPhone was locked to At&t and to my surprise this refurbished iPhone accepted my TMobile. Not sure how that happen.

I want to update to iOS 7 but am concerned I might lose my ability to use my TMobile sim. Does updating affect your carrier or cause the device to "activate" again?

Thanks

 

[dhlizard]

If it is IMEI unlocked, updating the firmware will not make any difference

http://iphoneimei.info

 

[dictoresno]

once a phone is unlocked, software will not relock it.

 

[oVerboost]

When you say it accepted your T-mobile sim, on numerous times? First time? Anytime?

But as has been said, if the phone is unlocked it should be fine after any update.