I have been beating my head trying to figure out why my phone won't register with the SIP credentials given to me from Phonepower. But before I contact Phonepower, I must point out a problem with the error message I'm receiving which is "Contact: URI too long (403)." Looking at the possible error messages for Zoiper, a 403 error is "Forbidden" and "URI too long" is a 414 error. I'm assuming "Contact" is "User name" in the app which is my 10 digit telephone number (according to the Phonepower SIP credentials), the length of which I don't think is the issue. I suspect what I'm really receiving is a Forbidden error.
So, in summary, the error number does not match the error description in the WP version.