New here.
Have a customer that just bought a new location.
Building came with a Panasonic KX-T7431 system with 5 handsets.
Only issue is 2 handsets do not have one of the CO lines.
The other 3 handsets do have all CO lines.
I don't normally work on Panasonic phone systems and was looking for some assistance.

Thanks in advance

Since a 7431 is a phone for a TD system, you might try searching for a manual for the TD-816/1232. When you find the PROGRAMMING manual, you ought to be able to figure it out.

Carl