That would make me wonder if the VOIPo number is getting call filtered?
I wonder if on https://secure.voipo.com/beta/mobile you register you cell number,
then as a test you assign the mobile number as your outgoing number at
https://secure.voipo.com/beta/misc then try calling, I wonder if it could ring her number?

It could be a temporary test - I could also be just sending you on a wild goose chase!