I can assure you that a three-line phone does not offer A-lead functions whatsoever.

Let me back up and describe what A leads are. Basically, they are the two ends of a switch contact that are closed when the line is seized. When the call is complete and the handset is restored, these contacts go open.

Valcom units rely upon this pair of contacts (along with a 5.1K 1/4 watt resistor in series) to detect that two phones have gone off-hook. In doing so, the resistance across the A lead connection contacts is effectively cut half when two stations go off-hook. This causes the page control to release the connection to the paging speaker(s), resulting in a private conversation.

Unfortunately, there's no way to perform what you are seeking without phones that provide a true per-station contact closure when the line is seized. While there are A-Lead adapters out there, they won't work in this application. Sorry for the bad news.

Ed Vaughn, MBSWWYPBX