Please note, to be able to take advantage of this feature, you need to be licensed for PSTN Conferencing within Skype for Business Online. If you are not properly licensed for PSTN Conferencing, you simply won't see the Dialin Conferencing information in the Meeting Details.
The below video is an brief introduction to this feature, and how to use it. Keep in mind, the PSTN Conferencing settings that have been setup in Skype for Business Online appear to be what is carried over for Teams.
I hope that the above video was helpful in getting you started with Microsoft Teams meetings that have greater capabilities!