However they could IM me and see my presence fine.
The customer has on-prem Lync servers. I am on Office 365 which is setup in a Hybrid with our on-prem servers. Other people in Modality who are on-prem (Response Group users) could see this person fine (you want a name - okay, it's Leon).
It should be noted that I used to be able to see the presence and IM with no issue, also, after I moved to O365 I could. Something changed later* Anyway - back to the story....
When Leon IM'd me I would get errors like this in my event logs:
504 Server Time-Out
ms-diagnostics: 27002;reason="From-Uri Domain is not in the receiver-tenant allow list";source="Office365ServerName.INFRA.LYNC.COM";appName="IncomingFederation";OriginalPresenceState="0";CurrentPresenceState="0";MeInsideUser="No";ConversationInitiatedBy="6";SourceNetwork="5";RemotePartyCanDoIM="Yes"
A search on that error didn't really bring anything up of value as it was talking about the whole domain needing white listing and that couldn't be correct as it was a single user issue. We tried moving to different PC's, different networks, investigated policies that Leon had but all came up nil.
The issue wasn't a big enough pain for Leon to want to spend too much time troubleshooting but eventually while discussing about their customers Office 365 plans a light bulb went off.
"Leon, have you got your user account in Office 365 as well"
After confirming he had it was as simple as turning off Skype for Business for his user account in the customers O365 tenant:
and we were back in business:
So what happened?
My account is in Office 365. Leon's account was on-prem. He also had an account in Office 365 but Hybrid was not setup.
Therefore, when Leon IM'd me, his client talked to his on-prem Edge, resolved the DNS for Modality Edge, and got proxied to me in O365.
However, when I attempted to IM Leon, my client talked to O365, who saw that there was a matching O365 tenant for the domain and sent the IM there. Simply turning off Leon from having an Office 365 Skype for Business account allowed the Modality Office 365 tenant to ignore looking up his details in the cloud, I found the customers Edge server and all was well in the world.
Simple when you know how!
*what changed? The customer got Office 365 but had not setup all the hybrid integration (as they didn't want to use it all at that time).