In hybrid environments, Teams-only users can not chat with on-premise Trusted Apps
Teams-only users can chat with on-premise SfB users.
Teams-only users can NOT chat with on-premise Trusted Apps.
Islands-mode users on SfB Online can chat with on-premise Trusted Apps.
Further, in the Hybrid environment the on-premise Trusted App SIP IDs can not be added as Team Contacts as the search fails. Trusted App Contact Objects are obviously in a different section on the on-premise AD compared to users. (In AD : Configuration -> Services -> RTC Service -> Application Contacts) and Teams somehow does not look there.
So, these IDs are not available to Teams-only users while they are available to SfB Online/ Islands Mode Users in the same tenant. The messaging path to the on-premise SfB servers is the same for both types of users, so this appears to be not a messaging issue but simply making these IDs visible/surfaced in the Teams client.
Many UCMA-based Trusted App/bots rely on capabilities that are currently not found in the Bot Framework/ Graph API/Calling API and Teams-only users need to be able to reach them.
Ironically, if the Trusted App is on an entirely external domain, then there are no issues. The ID is searchable externally and communication can proceed. This step-brotherly treatment is reserved only for Trusted Apps on one's own hybrid infrastructure.