Though you've probably been through this with support already...

What internet service do you have? What router model and firmware version are you using?
What ATA did voipo issue to you, RT31P2 or PAP2?
If RT31P2, are you willing to put it 'before your home router', at least for a limited time for troubleshooting purposes?
If 'before the router' is not practical or possible for you, have you forwarded any ports? I don't recall the exact ranges recommended by Voipo, but if call set-up is the problem, forwarding 5060-5061 (UDP) to your ATA should resolve that issue. (If you forward ports 10000 - 50000, you should catch any RTP port that Voipo would want to use, but the range probably doesn't need to be that wide.)
Does your router have a SIP 'ALG' (application level gateway) option in its firewall settings? If so, make sure that it is turned OFF.

Chances are, I've forgotten a few common troubleshooting points (besides the best range of ports to forward), and someone else may chime in with more points. Also, once you answer these questions, some more thoughts may come to mind...