The CUCMBE is ready to relate with PrivateServer using the SIP Trunk, but still it can't until we create the same configuration on the PrivateServer itself.
2.1
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Prerequisites
If you came here then all the necessary prerequisites are matched.
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The Virtual phone number is optional and can be left blank.
After you're done with the Inbound you have to to check that the Outbound section has either a dialing rule set to let the trunk be used or the trunk set as the default one.