How to achieve inter tenant \ domain use of MS Teams and Skype

Hello Experts,

I support multiple companies and am working to enable inter domain \ inter tenant use of Teams and Skype between some of them per the article listed at the url below.  Currently, I've made changes in the Microsoft Teams & Skype for Business Admin Center --> Org-wide settings --> External access area for each of the three, enabling "External access" and "Users can communicate with external Skype users" and then adding the other two domain names with status of Allowed.  After doing that I had some initial success in chatting with a user in one of the other domains when logged into the Teams app on my local system.  Since that time, however, I have attempted to chat with other users in the two other domains from my Teams app, with no success.  When I send a chat message, I typically get a message stating "This user is unavailable or offline.  We've sent an email instead.".  I even observed this when sending a chat message to another user, when I was connected remotely to his system, and was able to verify he was logged into the Team app on his system.  Is there anything else from a configuration standpoint that I need to complete?  Do I need to enable Guest access in Teams for each of the tenants?

Thanks in advance,

Russ

https://docs.microsoft.com/en-us/microsoftteams/let-your-teams-users-communicate-with-other-people