In the o365 Teams Admin Centre, navigate to Org-Wide Settings > Teams Upgrade and make sure Islands Mode is selected under Coexistence Mode - this allows users to use both Skype for Business and Teams apps whilst keeping the two products separate.
You will need to consider ‘external access’. You may only use external access in Teams (o365) or Skype for Business (VIA); not both. Should you choose External Access with Skype for Business (VIA) you will need to follow the below steps.
First, ensure External Access is disabled for Teams. This is a setting in the Skype for Business and Teams Admin Centre in o365. If you have users enabled for Teams and choose to turn on External Access in the Skype and Teams portal, only Teams will be able to federate with external organisations. Disabling this allows only Skype for Business on the VIA UC Platform.
For reference, see the image below:
In order to communicate with external organisations (domains) away from the VIA UC Platform by IM and Skype call or see their presence status and vice versa, you must ensure that no Skype for Business Online or associated products are enabled in your o365 tenancy.
Our preferred method of disabling these licenses is to connect PowerShell to Skype for Business Online and disable SIP. To achieve this, you will first need to download both PowerShell and the Skype for Business Online Connector. You may find instructions here:
Once you have connected to your Skype for Business Online server, run the following command where ‘YourDomain.com’ is your own domain, e.g. via.co.uk
Disable-CsOnlineSipDomain -Domain YourDomain.com
Note: In order to access the Microsoft Teams and Skype for Business Admin Center after this completing this process, you will need to log in with an account which has the appropriate administrative rights for your .onmicrosoft domain, as any accounts on your 'real' domain will not find an autodiscover record.