Almost every well known phones today are supporting intercom mode.
When a group of phone is in one way paging mode, it should be only a feature on the asterisk side to switch a particular phone in duplex mode, and cancel all other calls in the group.
To trig the switch from one way paging mode to intercom duplex mode, a simple press of a digit on one of the paging group phone could advertise asterisk through an applicationmap ?
This interactive feature would transform the PA like FreeBPX paging to a true Paging / Intercom system.
To allow for a greater flexibility, i think that it is important to take into account user rights. We should be able to decide wich user or device can switch from simplex paging to duplex intercom. It can be desirable to forbid some phones or users to reply to a paging announce.