I have in total 5 Grandstream phone, 4 DP720 cordless and this GRP2613.
I create the phone account on my fritzbox and configured in the DP 750 to manage the cordless, and directly in the GRP2613.
The cordless work since years without problems, the GRP2613 is only some months old. He did the problem since the first weeks, than i update the firmware and seems solved but after a while come back again.
The phone work perfectly in all the functions, it’s only the the ringing become mute after some days and only a reboot can solve.
I can wrote kilometers of details, but if someone have the same kind of problem, of if is a known problem, you can answer directly.