I have installed dozens of the Grandstream gxp2020 without any issues. I have 1 client with 10 of these phones and some of them stops ringing for no apparent reason. I reboot the phone and it works fine for a while.
The settings match all my other accounts which have no issues. They also have hight speed FIOS internet so it is not a bandwidth problem.
It is as if the DND button is pressed which is not the case and it shows online on the phone.