Enabling support for Office 365 guest and federated users in live events has been added to the backlog. We’ll update on the timeline when we have more details.
This item is still being reviewed by the feature team.
119 voteswjvii commented
They already added callto, im and sip although it would be nice to also have the option to use this for tel and a few others so that it would work with things like Dynamics 365, etc. See June 20th here https://support.office.com/en-us/article/What-s-new-in-Microsoft-Teams-d7092a6d-c896-424c-b362-a472d5f105de for details on the supported protocols so far.