I've been experiencing intermittent incoming calls going to VM instead of ringing on my side lately even though my ATA was online and registered. Seems to have just started recently and like you mentioned nothing has changed on my side. I've been using sip.voipwelcome and switched to sip-central01.voipwelcome over the weekend to see if that will make a difference but have had only a few incoming calls this week which worked ok, but not sure if this server is better or not yet since the issue is intermittent.

Which server are you using?