As our friends have said, it depends on the model of the Norstar.
The newer models (Compact Plus) have 1 in built ATA, but older models will require 1 ATA per extn.
The ATA's are well marked, one pair connects to the digital Krone connector & the other pair is the Analogue pair out of the ATA which you connect to the outgoing pair of wires to the telephone, fax, ans/machine,etc.