Have a system that has been running for quite a while. Running 220.127.116.11
When monitoring the system, is see all of the sip devices go unavailable, then unreachable, then unavailable, then idle, and finally reachable.
It appears that this happens approximately every 15 minutes, and the entire process resolves itself quickly.
No one complains of service interruption, but this is not a busy system. No one complains of internet loss. Phones and computers use one of two POE network switches.
I would suspect a network switch issue, but then both switches would have to be an issue.