Incomprehensible behavior of the grandstream gxp1620 phone, where to dig?

Question

SIP

Hello, since yesterday there has been an incomprehensible situation with the grandstream gxp1620 phone, the registration does not rise immediately, after picking up the handset and talking, the call continues to hang, after about 5 minutes the call is dropped, if you pull out the network cable, the reset occurs immediately. After about 1-2 hours, the operation of the device returns to normal, if you pull the power cord, everything repeats.
What to do with it and where to dig?

Answer 1

Is the SIP server on which registration is located on the same network or on the Internet?
Are there other SIP devices in the office?
What is used as a router?

Answer 2

Hello, I will continue to describe the problem for a colleague who is on vacation)
The problem is with any calls, we have asterisk and the phone problem is not related to it. About 150 phones in the network, all grandstream. We tried to create new local accounts and set up old ones on the problem device, the situation does not change. Factory reset and updating to the latest official firmware did nothing either. Even if you disconnect it from the network and start typing some numbers, they will start to appear on the screen in two seconds, which is not normal. What is most strange is that we got two such phones at once in one office. I would like to somehow flash them manually to some old firmware, but we don’t know how to get the root.