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
Should this not resolve the issue, you may need to consider ‘external access’. This is a setting in the Skype and Teams admin portal in o365 which determines whether you will use o365 for external access or the VIA UC Platform. 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: