Go to support.avaya.com to download the manual for your system, for free. The main ACS module connects the first 9 telephones (and/or faxes, modems, conference speakerphones, postage machines, etc.), and the first 5 outside telephone lines (voice, fax, whatever). The backplane of the carrier makes the electronic connection between the modules.
If is through programming that you make the lines appear at the phones or other devices, and also set the access and ringing parameters.