Your home or neighborhood must not have a strong Verizon tower signal? Do you have any neighbors on Verizon? They would have the same issue.
Turn on WiFi Calling under your cellular settings. Be sure to add your address exactly as shown on your Xfinity account. This will automatically route your cellular calls and texts through your router/modem/Internet to the XM/Verizon cellular servers when your cellular tower signal is poor or very congested.
From my conversations with Xfinity and Verizon about my poor coverage it is NOT the same as Verizon. Per Verizon Xfinity only lease space on the towers and may not lease space on the tower you need to get the same signal strength as Verizon. My Xfinity reception went to basically nothing when I moved. My neighbors have Verizon and the have a full strength signal,
Maybe you are in an extended coverage area that does not get shared with MVNOs like XM. Check the link above. Extended coverage areas/towers are most likely owned by another carrier and leased to Verizon.