How to unwind a mobile number transfer and send the phone back?

I'm writing on behalf of my 82-year-old aunt who was recently convinced by a sales agent to sign up for Xfinity Mobile and port her Xfinity Voice number to the new service.  She misunderstood what this entailed and was horrified when her landline lost dial tone.  She has no use for the new mobile phone because she already has an iPhone and can't make the switch to Android.  We just want to unwind all of this, get her Xfinity Voice re-activated with the same number that she had, and send the phone back.  How do we do this?