Hybrid Call Service Connect Extension Dialing and Media Failures
A wealth management firm has 4,200 users on Cisco Unified Communications Manager 14.2 and 800 remote advisors using Webex App with Hybrid Call Service Connect. The CUCM cluster is in Dallas; the Expressway-C/E pair is also in Dallas; advisors are in London and Singapore. CUCM uses 4-digit extensions 2xxx and DIDs +1 214 555 1xxx. Webex App users can see presence and chat but dialing a 4-digit extension returns “number not found”; calls to hunt pilot +1 214 555 1800 drop after 30 seconds. Remote advisors report one-way audio on calls to Dallas. Expressway-E is behind static NAT with one public IP; firewall allows inbound TCP 5061 and 8443; no TURN ports. CUCM has two subscribers and one publisher; no SME; the hunt pilot uses 180 ringing and early media.
1. Explain how a Webex App call reaches CUCM in Hybrid Call Service Connect and what is likely missing for 4-digit dialing.
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In Hybrid Call Service Connect, the Webex App does not register directly to CUCM. The call is anchored in Webex and routed through the Expressway pair to CUCM, where the CUCM call control and dial plan decide the destination. Presence and chat working proves identity and cloud connectivity, but it does not prove that the call path or dial plan is correct.
For 4-digit dialing, verify that Control Hub shows Hybrid Call Service Connect healthy and that the Expressway Call Service is enabled. Then check the CUCM side: the Call Connector user, SIP trunk or route to Expressway, and the translation pattern or route pattern for 2xxx. Webex App may be sending the digits as a URI or as an E.164 number; if the CUCM dial plan expects four digits, the call will fail with number not found. Confirm the Webex dial plan includes the internal extension range and that the CUCM SIP profile does not reject the incoming URI. Test with a DID first; if the DID works but 2xxx fails, the fault is dial-plan normalization, not media or registration.
2. Why would hunt pilot calls drop after 30 seconds and one-way audio occur, and what would you check?
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A 30-second drop with early media suggests the call is established at signaling but the media path or early media handling fails, so CUCM or the hunt pilot releases after a timer. Check whether an MTP or TRP is required on the Expressway-facing trunk, whether the SIP profile allows 180/183 with SDP, and whether codecs are negotiated consistently. If the hunt pilot uses early media, a mismatch between delayed media and the trunk configuration can cause the caller to hear nothing and the call to clear.
One-way audio points to Expressway-E NAT and media traversal. The firewall allows only TCP 5061 and 8443, but Expressway media traversal requires TURN and a UDP media port range. With static NAT and no TURN ports, the return media cannot reach the advisor. Verify Expressway-E static NAT, TURN server configuration, and that UDP 3478 plus the media range are open in both directions. Confirm the same issue does not affect internal MRA calls, which would separate a general NAT problem from a Hybrid Call Service-specific routing problem.
3. Recommend a numbering and routing design for the 800 remote advisors across London and Singapore.
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Keep a single authoritative E.164 space for CUCM users and map the 4-digit extensions into it, for example +1 214 555 1xxx with the last four digits matching 2xxx. Publish a Webex dial plan that sends 2xxx to CUCM as the internal extension range and sends +1 numbers as E.164. Use translation patterns on CUCM to normalize incoming URIs and to convert internal calls back to 4-digit extensions for display and voicemail. Avoid adding a second overlapping 2xxx range for London or Singapore unless CUCM has a clear site-based partition and translation strategy.
For London and Singapore, do not force all media through Dallas if latency and one-way audio are already visible. Add local Expressway pairs or at minimum validate the existing pair's capacity and NAT for those regions. If the business wants local PSTN breakout later, plan it as a separate gateway design rather than mixing it into Hybrid Call Service Connect. Document hunt pilot numbers as E.164 and test both early-media and delayed-media paths from each region.