The voice/video/meeting component of Microsoft Teams is built on the next generation of Skype for Business infrastructure (which is touted to bring about the unification of the Skype consumer and Skype for Business platforms).
There is a number of integration differences when looking at Microsoft Teams as well as Skype for Business Server vs. Skype for Business Online. In short Microsoft Teams does not talk to Skype for Business Server, only the Online version as part of Office 365. This is documented so should not come as a surprise, and hopefully will be addressed when the product is fully released sometime early this year, if not in the near future.
However, it is important to note with the below screenshots that I am using Skype for Business Online, but the other users are on-premises.
The presence display carries between both systems:
Users in Microsoft Teams are presented with a feature limitation warning on talking to someone on Skype for Business:
There is a nice icon in Teams to show the user that the other person is using Skype for Business:
Users receive a toast notifications from both Skype for Business & Microsoft Teams:
Where the conversation continues depends on which toast notification you click.
Skype for Business meetings show up in Teams:
If the user clicks on the Join button on a Skype Meeting from the Teams client – it will re-direct them to the web interface of Skype for Business and from there launch the client.