Migration from Lync hybrid to Skype for business online using own PSTN carrier

I’ve just completed a migration of a customer to Skype for Business Online, that’s right, Skype not Teams. The reason for this is that the customer wanted to retain their existing PSTN connectivity and currently it is only possible with Skype, using your own SIP is currently expected for Teams with a preview due at some point in 2018 but as yet no fixed date.

There are two real choices for PSTN connectivity with Skype for Business Online using your own carrier, maintain a hybrid deployment or use Cloud Connector Edition (CCE). In this case a Sonus Cloud Link appliance containing both a Sonus Session Border Controller (SBC) and CCE was deployed, in the future it will be used to provide SIP straight into Teams.

You might ask, why would they just not port/transfer their numbers over to Office 365 and use Microsoft as their carrier? This particular customer wanted to provide the capability of receiving PSTN calls to all their staff but realised that only a small minority would likely require the ability to make calls, to port all their numbers and use Microsoft as a carrier would require more calling plans than they would actually benefit from. Additionally, by using the CCE they are able to keep the voice traffic off their main internet connection to ensure the best call quality available to them.

Now that the approach and design is taken care of I just want to cover a few points encountered during the switch over.

Firstly, it’s worth noting that using CCE is only possible if there is no on-premises deployment, Microsoft reversed its decision to introduce support for that topology in 2017. Whilst configuring a user to use CCE for telephony Microsoft checks whether they’re a pure online user or part of a Lync/Skype hybrid deployment.

How does it determine this? And how can you check? Well you can use Skype for Business Online powershell to connect and check the attribute interpreteduser, for example:

Get-csonlineuser -identity user@domain.com | select interpreteduser

If the returned value is hybridOnPrem or hybridOnline then Office365 thinks there is a hybrid deployment. Now if there is no hybrid deployment as the users have successfully been migrated, the Lync/Skype deployment has been decommissioned and all relevant hostnames have been changed then how or why does it think there is still a hybrid deployment?

The answer is due to the existence of the msRTC* Active Directory attributes used by on-premises deployments, if a user is synchronised to Office365 with these attributes still populated then it will be automatically assumed a hybrid deployment exists and as the interpreteduser field is a system generated value it cannot be manually changed.

The solution is simple, remove all the msRTC* attributes and resync the user and wait, after Microsoft’s internal replication has taken place, which in my case seemed to be around 5-6hrs, rerunning the command returned a value of DirsyncedPureOnline and the user could be configured for CCE.

This could potentially be one of the reasons why it is suggested to keep the hybrid deployment if using your own carrier, however as not all customers want to maintain complex on-premises deployments I just want to reassure that it can be done.

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