I ran into "busy here" and while probably not the same problem, it may help.
My phone numbers are in 808 code (Hawaii) and
Voip.Ms treats that as International (for billing). Thus I have to enable International calls and allow something like North America.
My reason for mentioning this is that I didn't think I had made any changes, but it appears I limited international calling, which at the time didn't make me think it would effect Hawaii (being part of the US since 1959. :-)
So, yes, it may be something like Do not disturb or call forwarding or something.
Try setting up a different sub account. If the problem disappears, then it is something with that one suabbount blocking it.