It's a special cable, the plugs are thinner than regular cat5/6. You can get a free cable when You buy another SBM24, as Avaya includes them. Probably the reason I couldn't find a part number for just the cable!
They are hard to find, I got a few last year from CodeMicro, they carry replacement SBM24 cables.
Also If You have any blue keyed cables from older 500A Headset adapters/S203 Speakerboxes, they will fit & work on SBM24s!
Andre