Occasional ICMP timeout

Hi all,

We are having a totally strange problem with Grandstream GXP-2000 sip phones connected to our local Freepbx. We get occasional voice call quality problems, once or twice per month for example, where the voice calls drop, or crackle. At the same time the all of our Grandstream phones become unavailable.
They become unreachable or lagged and moreover, when they are pinged they show intermittent “Request timed out” responses although all the other devices on the network response with no problem.Firmware update didn’t resolve the issue. Currently we are using soft-phones(Jitsi) as an alternative without any problems. I would appreciate any hints. thx.

Well, It is completely random and unpredictable. I could not do anything except switching to soft-phones. I changed qualify to no but that did not help.

When this issue happens what do you do to fix it?
Does it happen at random times or is it predictable?
Is it always at the same time or on the same day?