Thank you for taking the time to touch base! I’ll be more than happy to investigate this for you. I wanted to find out what app you were trying to use given that the only application that we offer in where you can place a call from is the Xfinity Connect app which is tied to our residential landline service. Without a Comcast landline, you won’t be able to dial from the Xfinity Connect app.
Are you getting that error code when you launch the Xfinity Mobile app? Let me know of any other details pertaining to the situation, that way I can gather a clearer picture. Thanks again for your time!