Conference Call Automatically

I am trying to emulate the old style analog phones where picking up one phone would “join in” with an ongoing conversation. Is there any way, preferably an easy way…, to do that with freepbx/asterisk?

Just brainstorming, I think it might be possible to do it with a conference room that everything goes to, but that seems a bit convoluted.

Thanks for helping

Edit: I would like to do this with sip phones (ie. cisco 303) or ata bridges (ie. grandstream 286), but avoid wiring into the legacy phone system at my house.

Edit: Completely forgot about shared lines somehow. That is what I was looking for.