Skype 4 Business can't reply to MS Teams

My organisation uses Skype 4 Business and a partner organisation uses MS Teams.

We have open federtaion.

Our Partener org can us MS Teams to message us however we can't reply unless we instigate a new message window in Skype 4 Business

However we can instigate a messagein S4B and the partner organisation can reply from MS Teams and we can have a conversaton in the same window.

Telephony between the clients appears to be working.

Is there any guidance out there on this issue?