From the surface, it does not appear that your issue is the same as my issue, but what fixed the issue for me was turning "STUN" to "OFF" via the Web GUI on the phone. You could try it and see if that works for you. But since your firmware is later t...

Well, they told me it would be....If I get some time, I'll apply it to one of our phones and see if the phone continues to operate normally. 

Looks like v7.5.5 is still the latest firmware, but if you shoot me a message via my website below, I'll be happy to share the test firmware Cisco built for my SPA508G phones which has resolved the issue for my clients.www.TechAdvantage.co EDIT:  I'v...

@Christie:  In our testing, we just turned off STUN in the phone's web GUI and turned off DHCP option 66 so the phone did not auto reprovision itself. I may have also manually edited the provisioing file for that particular phone we were testing on, ...

I received a response from Cisco this morning saying they were able to replicate the issue and are putting together a beta firmware update for me to try...