I just posted a suggestion for freepbx 15 to have a command in EPM to factory reset phones. What would you like to see added in FreePBX 15?
This will help people like you.
I remember we had to do this once at a client with 40+ Grandstream phones, IIRC we wrote a curl script which logged into each phone, factory resetted the phone, and we obviously had DHCP option 66 activated, so as soon as the phone came back up it automatically provisioned with the correct extension. Good times…