@James1 I have not read anything regarding this topic in the knowledge base article. Also, I didn't come across this issue on SFBS 2019.
Some quick steps/questions regarding your issue:
- Is you federation working in general?
- If you enter a complete SIP-URI from an external contact, can you add it to your contacts or chat with him/her?
- Are all external access policies alright and allowing the external/federated communication?
- Did you ensure that your firewalls, proxy, certs, (internal/external) dns ... are ok? (exceptions/rules)
- Maybe you could open another thread (if not already done) to dive in deeper or raise a ticket at Microsoft Support (if not already done)?
