Blocking calls based on the call source allows precise control over where traffic is allowed to go.
By blocking any calls originating from the B2B traversal zone toward Cisco UCM, the engineer prevents B2B traffic from reaching UCM while still allowing internal video endpoints to route external voice calls through the appropriate zones and search rules. This preserves external calling functionality without exposing Cisco UCM to B2B traffic.

Using Links and Pipes to Manage Access and Bandwidth.
You can also manage calls from unrecognized systems and endpoints by configuring the "links" and "pipes" associated with the Default Zone. For example, you can delete the default links to prevent any incoming calls from unrecognized endpoints, or apply pipes to the default links to control the bandwidth consumed by incoming calls from unrecognized endpoints.
https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/expressway/admin_guide/X14-0-2/exwy_b_cisco-expressway-administrator-guide-x1402/exwy_m_zones-and-neighbors.html#reference_AB9714D9A58AB2411A667F451A8ED9A5

Webex video quality depends on sufficient prioritized WAN bandwidth and correct DSCP classification. The policy prioritizes traffic marked AF41, so the engineer should confirm that Webex endpoints are marking video traffic as expected and increase the VIDEO class priority allocation if the current 20% is insufficient during fluctuating uplink conditions.