It looks like some people in the thread below have figured it out — we actually have this. You can start a chat both ways; the system will automatically recognize if this person is on Skype for Business or on Teams.
Thanks and let me know if you have any questions.
SuphatraMarius Motiekaitis supported this idea ·Marius Motiekaitis commented
No, we do not have THIS!!! There is no way to IM other user to Sfb if he has Teams.