It's just a case of elimination but have you checked if both of the IMEI numbers are blacklisted or not? I don't expect them to be. I know a 3rd party has checked the phone but I rather get that eliminated. I know this has happened after a firmware update but its just a case of checking... there are free blacklist checkers online.
Also, a 3rd party repairer could possibly void any warranty if they have opened the device up.
If after checking the IMEIs are both clean you may have to contact the 3rd party to see if there is anything they can do too.