Cisco SPA508 acts busy when open


We are a small business IT firm and have successfully had Asterisk for a year or so now, I must say it is pretty awesome.

The only MAJOR bug we have encountered is with our Cisco SPA508 phones, at least it seems to only be that model. We also use the 504s and 512s, and neither of them experience any issues. So basically the issue we have is that about once a week, the phone will act busy or as if DND is enabled. I check the asterisk logs and it seems that the phone is refusing the call, but it does not register as busy when I am not trying to call the phone. Is there some signal asterisk needs to send or something? The phone dials out fine, but it cannot be called internally and it does not ring when external calls come in. At this point I think its the phone, anyone else with this phone experience the same issue?