Partyline, IFB Expansion panel
Clear-Com offers a range of partyline IFB products
AX-704 (current)
4 channel IFB Expansion Control Panel - requires MA-704, Can be fitted in a console or optional rack mount
Obsolete unit AX-4
Each AX-704 and AX-4 is intended to access IFB 5-8 and up (in groups of four)
Each AX-704/AX-4 controls it's own additional IFB controller PIC4704
http://www.clearcom.com/product/partyline/program-interrupt