There are lots of reasons that a phone can't break dial tone.
First, we need to know more information and what "version" of CBDT you are reporting:
1. I hear tones, they sound correct, but CBDT
2. I hear tones, they don't seem to sound right.
3. I hear wavering tones as I push the buttons
4. I hear a "tink" sound when I push the buttons
5. I hear a click when I push the buttons
additionally...
6 all the lines on the one phone CBDT
7 some of the lines work, but some CBDT
8 I have more than one 2565 and this one CBDT but the other(s) work OK
9 the phone is / is not a WE phone
10 I have checked the polarity, voltage and current of the line(s) in question
11 I have connected a butt set directly to the line(s) at the
a. NID
b. KSU input T&R
c. KSU output T&R
d. T&R terminals on the key in the telephone
...and here are the results of these tests:
So, as the members above have suggested, before you start moving wires around inside a factory-wired, unmolested phone...
(and what do you mean by "...the terminals are connected to the board of the phone so I would have to cut the wires." ? The mounting cord conductors in a typical 2565 are terminated with spade lugs, which are fastened under the screw heads on the key,)...
I would suggest that you do your diagnosis in a scientific and methodical manner.
But tell us the answers to the above sequence of observations. The answer might be as simple as a dirty contact on the common switch of the dial. I have been repairing 2565 sets for 52 years, so I'm confident that I can help you.
JUSTBILL's suggestion is the least invasive and would be my first choice, if I didn't have a butt set (or a 2500 set with test leads) handy. Installing a polarity guard is not a repair. It is a work-around that is not necessary if you follow the logical trouble-shooting scheme explained above.
