This is down to how cisco/webex resolves DNS. Depending on config it can be looking for external DNS records which need to resolve.
We already had an internal AD domain added (customer.com) in the umbrella dashboard as part of the internal domains.
On a working cap we saw it look for
_collab-edge._tls.customer-ext.com which didn't resolve
Then it looked for
_cisco-uds._tcp.customer-ext.com
which did resolve to the customer ucm server IP's (this must have been resolve by internal DC/DNS)
On the "not working" capture
Wireshark showed a SRV record
_cisco-uds._tcp.customer-ext.com was attempting to get resolved by the client but getting a "No such name" back from a public DNS server. It tried to resolve on the internet.
We needed to go into the DC and edit DNS on the external domain customer-ext.com
We also added the customer-ext.com to the internal domains in the umbrella dashboard.
Find DNS queries that didn't return an answer
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