It uses MGCP, you will have a world of hurt getting anything but a very basic phone out of it in FreePBX (or even the underlying Asterisk) and you will have to do all that outside of any GUI.
You will have a world of hurt making Shoretel do anything they don’t charge you for, consider porting your DID’s (no PRI’s required) to someone more incompetent and just eat the cost of replacing their outdated hardware, it will be cheaper and better even in the short run . .