Incoming VOIP calls were intermittently not working to registered SIP phones. The problem was caused by registration data not syncing correctly between RADIUS servers and has been fixed. It would have resulted in some calls going to voicemail even when the callee's phone was successfully registered. Sorry for any inconvenience.
Coincidentally one of our carriers also has had call issues during, and after, we fixed our RADIUS problem which has caused a bit of confusion!