The ability to make operational use from listening to adjacent atcs was new to me. This is not possible where I am because there is no way to tune a receiver to another than a preselected frequency AND to put the signal into the control room.
121.5 being the emergency frequency where all airlines and at least one person in every ATCC
should be listening to, it is quite strange that where you work you don't have access to it. Because as I understand, may the force be with me said he heard the sofia call the aircraft on 121.5, and if their antennaes are within range and contact I don't understand your doubt in him being able to hear the transmission.