The part you want is made by Viking, called the ldb-2.
It can be used to detect ringing and or in use. I would suggest avoiding the ir trick, however. You will probably have greater success by using some sort of voltage activated ab switch (read Niles SAS-1) through the tape monitor circuit. With this scenario, you would ALWAYS leave the tape monitor on, and loop the signal through the sas-1. When voltage hits the sas-1, it would switch to the priority inputs, which in our case would be open. Muting occurs, and no ir bugs to worry about. It would be cheaper then using a contact close ir generating device.