The way I did it on my environment was slightly complex and with Skype, but bear with me...
It all depends on putting a prefix on to the person's number on the Microsoft side and if you're using E.164 extns on CUCM (as I was)
Firstly on CUCM we were already using E164 numbering plan (so 1xxxyyyzzzz for US users, 441xxxyyyyyy for UK users etc) but we didn't use + prefix as its a pain to dial a + on a standard Cisco phone.
Let me use 441999888777 as an example.
The Cisco phone has 441999888777 as a DID line and probably has just 8777 as a shortdial.
Configure that user on Skype/Teams with a number +00441999888777 (I know it's sort of invalid looking but it should take 14 digits on Skype or Teams without issue)
Create a SIP trunk between CUCM and Skype or via an SBC to teams.
Now, on Cisco side, add a pattern *1441999888xxx to send the calls towards Skype/Teams, but add a translation for called numbers to remove *1 and replace with +00.
Now your calls diverted or forwarded to a normal extension with a *1 prefix go towards Skype/Teams.
If you're using SNR, the person's alternate extn would be their normal DID with a *1 prefix, or if you just forward all calls to site you could do this via s simple translation for all inbound numbers to add a *1 prefix - this would then be picked up by a route pattern and sent to the Microsoft side.
When the *144xxxxxxxxxx version of the DID is called it therefore goes across to Microsoft side as +0044xxxxxxxxxx
Skype/Teams should accept that and ring the Skype/Teams extn.
When someone on Skype/Teams calls another person on Skype/Teams, they use a URL which would send the calls within Skype/Teams, and the strange +00xxxxxxxxxxx number wouldn't be visible.
When someone on Skype/Teams then calls an external number or phone extn instead, you route that number towards the SBC(if Teams) or to CUCM (if on-prem Skype). As the calling number comes across the trunk, you remove the +00 from the calling number so it becomes a normal DID extn or external number and allow the call to the extn or out to PSTN.
I did this on an environment covering about 10 countries, and it worked well for us, but it only works as Skype at the time allowed >=14 digit extensions, so that was our 11 digits plus a bit of a prefix. The end users seemed oblivious mainly to the config as they called each other by 'name' rather than by extn, but if someone on Skype dialed a shortdial extn in their country, we normalized that back to a full E164 number and sent it off down the trunk to CUCM. There was a bit of hair-pinning of calls if someone did unusual call forwarding, but in our case that was about once a year out of 5000 users.
I would suggest you take a look at your dialplan first though to see if your number plan would support this, as if you're using a short dial plan (i.e. 6-7 digit extns) you might need a more complex set of translations than I had.
David
