I dont believe it's possible, not only will you have custom firmware you also have what is called GAPS which is the grandstream provisioning server the phones automatically reach out to each time it's rebooted.
Best bet would be to tell them get new phones
I dont believe it's possible, not only will you have custom firmware you also have what is called GAPS which is the grandstream provisioning server the phones automatically reach out to each time it's rebooted.
Best bet would be to tell them get new phones