I have a Kx-t7420 panasonic phone system.
Actually, you have a Panasonic TD phone system and a 7420 phone, but no matter. The feature you have is probably "idle line preference" or "preferred Line Assignment-Outgoing". In TD systems by default, you select lines from highest to lowest. If you only have 2 lines and PLA is set, you get line 2 then line 1. You can change it to get line 1 first, or chage the trunk access order. The first is something you can do from each phone, the second requires access to system programming. The bad thing about the first solution is if line 1 is busy, you won't get any line. The second will give you the entire trunk group.
Anyway, from each phone without picking up the handset, press PROGRAM 99 and dial 1301 STORE. That will give you line 1 when you pick up the handset. Just so you know or can change it, 11 is none, 12 is idle line, 13xx is specific line and 1+the intercom is intercom. Pick up the handset and drop it back down to exit programming.
As far as the noisy line, it's probably a central office issue. It's hard to test as that line is the outer pair of the jack that plugs into the phone sytsem (RJ-14 pair 2), but you can probably take a regular phone and test it at the demarc point.
on Jul 14, 2010