Okay, this has been going on for two years. I've tried many, many things.
What's happening: we have 10 Cisco phones, the users lines occassionally go orange (not available) for a few seconds to many minutes, usually self-resolves but sometimes a reboot is required. Only happens to one phone at a time usually. Rarely a phone will reboot itself. I have changed out every part of the chain: new router RV345P, phones directly connected to the POE ports. Phones: we've gone through SPA525G2, SPA509G, and the new 850 and 860s. All have the same problem. We have not experienced a dropped call in the middle of a call, only happens when phones are idle.