Ask them to check their Coexistence mode. Looks like they maybe have “Island” mode on. If they have this, it explains why it isn’t working as Teams messages stay in their Teams island. If the end user is running Skype for Business and haven’t turned on Teams, they receive nothing as the message stays in Teams (island mode).
As them to set the coexistence mode, for that particular user, at least on SfB only/collab or collab Meeting mode. It will take something like 24 hours before all has been set on backend with Microsoft.
Let me know if this was the case :grinning_face:
Kind regards and stay healthy,
Mitchell Bakker