My company has a very restrictive policy regarding IT security; most cloud solutions are forbidden. We have an on-premises Skype for Business instance.
In one of our projects, involving only a small subset of the company employees, we work with some partner companies which have Skype for Business too.
For improved collaboration, we'd like to be able to chat over Skype with the employees there.
Is there a known way to achieve this? Microsoft Documentation is not telling much. I don't ask for precise configuration instructions, but for the shared experience of the community at the concept level.
Restriction: the partner company shall only have access to our project team, not to the entire company's employees.