Coexistence and interoperability between Skype for Business and Teams clients is controlled by coexistence modes.
The coexistence mode of the recipient determines the behavior of chats, calls, and presence, both within a tenant and across federated tenants. 

Users in our tenant currently making and receiving PSTN calls in Teams are in TeamsOnly Mode.
Users in our tenant who have never been assigned a phone number for calling are in TeamsOnly Mode.

Federated means any chat or call that is to or from a user not within the UIUC Tenant. (This includes communication to UIC and UIS)

The tables below capture routing of federated calls and chats, and are valid for new calls or chats. They describe which client will receive a new call or chat, if originated by a user on the left, to a federated target user on the right. In summary, if the conversation is possible as described above, messages sent to TeamsOnly users will always land in Teams; messages sent to Skype for Business mode users will always land in Skype for Business; messages sent to Islands users will always land in Skype for Business regardless of the client from which they were sent.

Routing for federated chats and calls differs from in-tenant routing in that Islands users will always receive a federated communication in Skype for Business. This is because the federated partner may not yet be using Teams. Routing to Skype for Business for any islands mode recipeint ensures messages will always be received. Routing to Teams could potentially result in missed communication if the intended recipient does not use Teams.

Federated new chat or call routing to a TeamsOnly mode recipient (Where Sender is a UIUC User)