When an ARA connection is established (or being established when answering or making a call), ARA prevents other application programs from using the serial port you selected in the Remote Access Setup control panel. In this case, most programs will inform you that the port is in use.
If your Macintosh is set up to answer calls (the "Answer calls" checkbox in the Remote Access Setup control panel is selected), then other programs are allowed to use the serial port. When they are finished with the port, ARA re-enables your Macintosh to answer calls.
ARA Personal Server provides this port arbitration through a system extension called the Serial Port Arbitrator located in the Extensions folder. Some programs that use the serial port may not be compatible with this extension. If you suspect this, remove the Serial Port Arbitrator file from the Extensions folder, restart your Macintosh, and try the program again.
NOTE: The Serial Port Arbitrator is not needed with ARA Client software. If you remove the Serial Port Arbitrator and use ARA Personal Server, you must either select, or deselect Answer calls in the Remote Access Setup control panel. Also the Serial Port Arbitrator will not work on a Macintosh if ARA Personal Server has not been installed.