It's very important that you use a Teams Room license and not a Basic License. The Teams Rooms license it's equipped with all the features needed to run a Teams Room and more cheap than regular user's licenses.
However, it's possible to setup a meeting room with a Basic License. The most basic requirements is a mailbox and Teams. You'll just need to configure it properly to auto-accept invites and other parameters.
Give it a try and don't expect too much because malfunctions can happen without the right license. Keep in mind that is a "resource" and "room" type of "user".
Yes, there is a trial available at your admin's panel.
I don't know very well how the audio conf works but i expect that this "user" or "room" won't have a calling number to dial.