I have to admit three facts you may want to take a note of:
1) in order to hear error messages from Callcentric - your internet connection must have an adequate available bandwidth and the codecs of your Zoiper SIP client and Callcentric's servers have matched - otherwise you'd not be able to hear anything or the voice would be severely distorted;
2) the error messages you received clearly spell that your SIP client (Zoiper) has sent WRONG numbers or at least - in a wrong format; for example - some digits could be missing, etc;
3) both #1 & #2 also clearly spell (to me) that [to some great extent] you don't have a NAT problem - otherwise you'd not hear anything.
You may try dialing Callcentric test number 1-777-1234567 (no dashes between numbers
) - if you get the same error message as above - then it's clear that your phone misdialed the number. Otherwise you'd be connected to Tell-Me information system which you may control by dialing DTMF tones (keys) and/or by voice.
You may also call into your own Voice Mail box (make sure you "purchased" one on your account - it's free) - VM is activated by dialing *123 or by dialing your own account's number. You'll be able to hear and follow prompts as well as record messages or greetings and listen to you recordings.
About Callcentric - whenever an account is set to troubleshooting mode - their system keeps recordings of all signalling passed between your device/soft-phone and their servers. Having a trouble-ticket open with their support would probably already have your issue on the way to be resolved. The support hours are 8:00AM till Midnight Easter time.
Oh, b/w - there is NO such thing as NAT problem when working with Callcentric network unless a customer has a special kind of SIP-hunting firewall or buggy SIP router-helper (some were encountered in past, but not any recent cases). No STUN configuration is needed either. On some rare occasions there are known cases of highly asymmetrical bandwidth provided to a customer by their ISP - it was quite sufficient even for G.711 codec on download (you hear voice undistorted) while upload bandwidth was very limited, below any acceptable limits (less than around 100Kbps which is required for normal U/a-law G.711 codec).
You're welcome to send a PM (you need to become registered to this forum) to me if/when you may want any escalation of your trouble-shooting. Somehow I believe that your issue is NO different than any other of millions already resolved for Callcentric customers.