I have a business phone system that has 4 lines. Three of them ring from line 1 to line 2 to line 3, meaning is 1 is busy it rolls over to line 2 and 2 to 3 and so on. I just changed phone service providers and now my #2 line rolls into my fax line. How should these three lines be wired together.

The phone company makes the change, generally. Software.

You phone system may be able to do it too.

To add to above, your 3 lines go into the KSU separately, Usually with a Line Cord, or punched down on a "66" block.

Basically, I f you have a "phone system", it can deal with it.

If you have "3 lines" and a keyset to select the lines, then the phone company deals with it.

If you changed, then the phone companies "If busy" may be in addition to the "phone system's" keep busy to do requests.