Press line button to pickup call

im using 6757i. I have lines 1 and 2 set to an internal extension. I have lines 3 and 4 set to an extension that rings when an outside line calls.
I want to pickup lines 3 or 4 ONLY if they press the line button.
I cant figure out what to put in the mac.cfg to allow that?

any ideas?
**edit**
the issue is they get so many incoming calls that when somebody transfers a call to a desk or they pickup the receiver to call out they end up picking up an inbound call from outside. This happens multiple times per day. So I want them to see lines 3 and 4 blinking and ringing (as they do now) but picking up the handset should not answer those lines.