Why can't I use my Motorola MT8733 for Home Phone?

I want to add Home Phone Voice service to my existing Internet service where I use my own Motorola MT8733.

I placed Order Number: [Edited: "Personal Information"]  for Home Phone. I missed several calls that went to my spam folder, but eventually called 877-392-1494 and spoke with a representative, who told me I would need to rent an Xfinity modem in order to use Voice, despite my modem being listed first on the hardware compatibility page: https://www.xfinity.com/support/devices/#auth.

Can someone explain why I can't use my own modem?