Event ID 1400 - UM Server

I recently set up Lync 2010 at our offices and ran into a snag with Exchange 2010 SP2 UM voice mail integration. The symptom was the following error in Exchange 2010 UM:

The following UM IP gateways did not respond as expected to a SIP OPTIONS request.

Additional symptoms were that Lync client are unable to call voice mail and users are unable to leave a message for Lync users.

The Culprit

There are 2 possibilities here...
#1 - And perhaps the most common issue. 
The Subject Name of the Certificate assigned to UM MUST be equal to the FQDN of the UM Server.

The certificate assigned to the Exchange UM Server Role was not using its own FQDN as the Subject Name (SN), instead the SN was mail.domain.com. Sometimes I come across deployments where the self signed cert is added as the host name and not the FQDN, this too results in the same error.

Resolution
Re-issued a cert with the FQDN of the UM server as the subject name.

#2
The UM IP gateway in Exchange 2010 which is set up by running the ExchUCUtil.ps1 script doesn't have a port listed in the configuration. This can be viewed by running:

Get-UMIPGateway | fl

Resolution

Run Set-UMIPGateway -Port 5061