I completely agree with you about credit card issue - I gave them virtual number that expires this month and did not setup landline or cell phone with them. I thought it is NOT necessary in order to get calls on 775 number. But you are right - in order to use this service, backup "hard" number is needed.
I will try IPKall, as you suggested above after I cure my tablet - it died this morning for no reason - I will open a dedicated topic in Troubleshoot section.
BTW, what POTS stands for?