Mobility - Topology Doesn't Have Required SIP Listening Port Setting

I had a client with a Lync 2010 deployment recently phone me and tell me that some users were unable to login to their mobile client when outside of the organization. The first thing I did was check the MCX service on the pool that the users were homed to. I was immediately greeted with Server Error in MCX application:

I attempted to recycle the external MCX application pool which typically fixed this issue, however it did not resolve it. I then started digging through the event logs on the server and noticed the following error Application Event ID 1309:

To which the following portion of the error specifically caught my eye:

Exception message: Topology doesn't have required sip listening port setting

I then ran:

Get-CsService -Identity WebServer:FQDNOFFEPOOL.com


Which showed that the value for McXSipExternalListeningPort was $null:

Then I ran the following to set the MCX External Listening and Primary Listening ports to the correct values:

Set-CsWebServer -McxSipExternalListeningPort 5087 -McxSipPrimaryListeningPort 5086 -Identity WebServer:FRONENDSERVERFQDN.com


Then running the get-csservice command again showed that the values were correct:

At which point I invoked CMS replication, verified it had completed replicating and then performed and IISReset and the mobility service was working once again: