Why does Xfinitiy not honor their call back appointments when trying to cancel my contract as I have left the country

Xfinity, this is now the third time an appointment for callback has been missed by your company.  As indicated in my many correspondence to you, I have left the country and wished to terminate the mobile service.  Despite hours of contacting and writing formal requests, you constantly indicate that I need a call back which you then appear to avoid - this is unacceptable.  I am going to stop further payments in an attempt to gain you attention and bring this to a final close.  Thank you for your commitment to closing the account.