Bridge mode removes a layer of translation between your phone and the Internet. When bridge mode is off, the Xfinity modem is performing "NAT", or Network Address Translation, between your modem's public IP address and the Xfinity's LAN network. By default, the Xfinity modem also performs basic firewall functions.
When you have an Eero, or similar product like a non-Xfinity WiFi router, the Eero is also performing NAT between the Xfinity LAN and the Eero LAN, and it also has its own firewall. So you wind up with "Double NAT" and a double firewall which is not only a potential performance issue, it can confuse network applications, like the iPhone's WiFi calling feature, which are used to being only one "hop" away from the Internet.
Bridge mode turns off the Xfinity modem's NAT and firewall, vastly simplifying the network topology, and letting the Eero have direct access to the Internet.
So my theory is that removing the extra firewall and NAT is helping with the dropped calls issue.
I hope that helps!