Microsoft sells a license called audio conferencing. It comes included with certain User types - generally E5 and A5 as of this writing. If a Teams User has the Audio Conferencing license assigned it has one set of implications for calling and if not there is another set of implications.
Microsoft User with Audio Conferencing add-on
If the Microsoft Teams User has the Audio Conferencing license (as seen below) there are a few behavior options that affect calling.
SCREENSHOT OF MICROSOFT ADMIN > USER > LICENSING
Add a Caller using Microsoft display numbers: If you want to add a caller in an existing call coming in through the PBX and you type in a Phone Number in the Add Participant screens in Teams. The call will be carried by Microsoft. The added caller will see the assigned Microsoft Number. These settings can be managed in the Teams Admin Center > Users as seen in this picture below:
2. Add a Caller user your PBX/SBC display numbers: If you would like to have the additional call leg go through your PBX/SBC you can switch the settings in Teams Admin > Users > Edit Audio Conferencing to "Don’t Allow Dialouts" for the User as seen below. This will push the Add A Caller function to use your PBX/SBC for the additional leg. It will also not allow the User to dialout to PSTN numbers to add in callers in a Teams Meeting.
Microsoft User without Audio Conferencing add-on
If the Microsoft Teams User does not have the Audio Conferencing license the default route when adding another Phone Call to an existing call should be the Direct Routing trunk that TeamMate has built to connect to a PBX/SBC.
In this case the User will not have access to Audio Conferencing when using Teams Meetings - no method of dialing in PSTN numbers to add users or providing an external PSTN number for accessing Teams Meetings.