I have a customer where external communication (with other O365 tenants) worked without problems. Since a few weeks most of the users are not able to do this. When you want to send a message to a user of the customer Teams says "mailadress not found" (i dont know the exactly sentence in english because i just know it in german).
I find out that these users do not have the attribute SipProxyAddress in Azure AD. Some users of the customers have, most of them does not have it anymore. I can add a SIP: entry in proxyaddresses on prem to get this done or enable the skype for business online license.
But how is it supposed to be right now? Was there a change that every Teams users needs a Skype for Business Online license to get a SipProxyAddress to communicate externally?
Skype Online License for using Teams has always been a prereq for interop to allow the gateway to function from what I recall. Are these users that have been used before that just stopped working, or are they just getting it lately from reaching out to certain customer/clients?
Skype Online license is required for Teams to external communicate since it currently rides over the Skype for Business infrastructure. That might change when they launch Teams Native federation (Which should be launching soon).