AT&T RELEASE 2 - Manual (Page 22)

FIGURE 15 The control unit connected to labeled outside lines.
NOTE: If you’re in an area with frequent lightning activity or severe
transient voltage and are using the 147A Protector (see CIB 3109), the
outside lines from the control unit plug into corresponding jacks on
the protector itself.
18 Step 1: Installing the Control Unit