When I first tried i-p-tell a couple of weeks ago I had a problem with voxee.com not working. Something was going wrong with authentication -- possibly because they don't accept REGISTER requests and authentication is done after INVITE to make a call.
I was trying to recreate the problem today (I've been on vacation) and now it's working fine. Hopefully my original problems were temporary as voxee is my favorite trunk for outbound longdistance.