iPhone 6. Can’t change voicemail password.

Spent hours with Mobile tech.  Set up iPhone with xfinity mobile 2 months ago. Never had an option to create a voicemail password. I can only dial in where I am asked for a password which I don’t know.  Tech tried resetting but nothing’s works.   Note:  when I hold 1 to access voice mail, a persons name comes up, maybe the previous owner as I bought this phone used.