Currently Teams Federation is provided through Guest access. There isn't much published collateral on the list of features (i.e. SIP to SIP or SIp to Other) defining what is required to build Federation with Partners especially where partners are on Skype and enterprise customer could be fully on Teams. Documentation is needing to provide customers guidance on how would you go about configuring with an OnPrem solution or what is needed to support federation in general. Full Federation guidance documentation is needed for these scenarios.
Mark East commented
This is the one thing keeping my organization from migrating to Team from SfB-- We can't federate with other companies who run SfB. What's the hold up on this functionality?