Voicemail Settings

I want to change the number of rings before voicemail answers, but cannot do that.  Tried to do it, followed your directions (top screenshot) but every time I get to call forwarding, the advanced call forwarding option doesn't work and says "Sorry, but something went wrong. Please try again in a few minutes." (bottom screenshot).  I have been trying for waaaaaaaayyyyyyyy more than a few minutes.  So if I can't change the number of rings, then I want to completely disable voicemail.  Didn't see that option anywhere, so I called Comcast, finally reached a tech, who told me to dial *99, and then press 7.  When I did that, the response I got back was "Sorry, that is an invalid choice." So then I chose the option to change administrative settings.  No option there to change number of rings. Have I ever mention how your customer service is the worst ever? 

Look, I either want to change my number of rings.  Short of that, I want to cancel voicemail altogether.  However, if that is the option, then I don't want to lose the Call History feature.