If I have 3 MD110 ( A,B,C) calls come in on PABX "A" if no one is logged in they go to PABX 'B' .If no one is logged in then calls go to PABX "C". My question is , do I just need to set up a data link (VIA ICU) between A and B and another data link between B and C . Or as well has having those links do I need another link between A and C