An auto answer speaker phone would work for one application, if you can find one for under $100. I was considering using a device that would connect to the audio and not have to build ring detect, pulling loop current and sense call termination.
The OBI has very clear signaling on the loop. Off hook and call setup (dialing) is at one polarity and the call content is at another polarity (they changed the default for this in the latest release). Once the call is answered a simple diode and resistor will keep the line open until the call has terminated.