You will need a TEL-14 interface connected to each frame.
1. On the "Setup" window and under ports, enable the party line check box for the tel-14 port on both systems.
2. Assign a talk and a listen key to the panel or panels that need to talk to the remote system.
3. You can create a speed dial to reach the remote matrix, to do this, go to "Setup'->"Attachments"->"DTMF sequences" and the telephone number of the remote system and check the box under the "available" column.
4.In "configure"->"Attachments", select the tel-14 interface from the list and attach the DTMF sequence you have created
If you need to add interfaces to the party line with the tel-14:
5. Go to "Configure"->"Local advanced" and add a force listen from the interface to the tel-14 and from the tel-14 to the interface