Well it happens to me with, so far, one number. Here's what's happening and why I don't see how it can not be on Voipo's end.

I call number (AT&T Wireless cell phone - area code 661) from my softphone (Bria iPad edition).
No ringing, goes to voicemail after several seconds of quiet.
I turn off WIFI bypassing router (and ATA) and use Verizon data network on iPad. Call number again.
No ringing, goes to voicemail after several seconds of quiet.
I switch dialing out account from Voipo to Callcentric (they have pay-as-you-go plan) in softphone. Call number again.
Rings! Goes to voicemail after several rings.

Clearly the no-ringing is happening when I dial out using Voipo and not when I use Callcentric. This appears to have nothing to do with the ATA or router. Any thoughts?