What failure modes do you experience with your VOIP carrier?

Folks: I'm curious to know what are the more common failures you see with your VOIP carrier. I have seen several from my carrier, and I'm curious to know what are the chief technical issues people observe. I'm not interested in billing issues, porting issues, or customer service issues. I'm looking for voice-quality issues. What I have seen so far are: a) Total outage of the system for all phones (no calls in or out), with all phones at least giving green status lights (Linksys SPA942 phones); b) 'slow to ring' outage, where the outbound call takes unusually long times to induce ringing in the _called_ phone; c) 'reboot' outages where the phone displays an amber status light (indicating a problem) and no calls are possible until rebooting the phone.