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 Grandstream phones become unavailable, that is, 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.