Good afternoon @user_9goybn, and thank you for reaching out on our Community forums, we appreciate your time working with us here! I have included the following link with instructions on how to manage your voicemail features https://www.xfinity.com/support/articles/manage-voicemail-features-in-xfinity-connect. Please let us know if this helps! 

I've been looking for an answer to this question myself, and after hours of searching I was able to find a solution in the following post:

https://forums.xfinity.com/conversations/home-phone-features/change-my-voicemail-number-of-rings/646e6d2527f9157f6ff948fc

While it will not disable voicemail, it will extend the number of rings which should allow for your answering machine to pickup.  Please note that this solution will only work if you have the Advanced Call Forwarding feature.  From all the posts that I have looked at there is not a setting that allows you to simply "disable" voicemail.  In order to completely "disable" voicemail it seems like you have to have the voicemail service removed from your Xfinity Voice features, which could introduce more complications.