In certain situations, you may not want to use the normal handset and hook for a VCom station. For example, what if a user wants to use a speaker and microphone at a particular station?
If you don't wire a handset hook into the VCom-VC16K, the VCom acts like the handset is eternally hung up on the hook. This means audio will never flow into the station.
To remedy this, you must jump pins 3 & 4 on J24 (which is where you normally wire the handset hook into). By jumping those pins, the VCom-VC16K will now act like the handset is eternally picked up/off the hook. This will allow you to always be active on a partly line. If you have a speaker connected, you'll always hear the party line audio. If you connect a mic, the mic will always be hot unless you put a button in between to activate/deactivate the microphone.