To expand on Dasheen's post, to get into line programming, on a set, press f**266344, pswd, show, show, show, then scan to the line that's programmed on that button, and press chg until it says unassigned.
Or use f*3, and program it to a different button. F*3 won't program over a line appearance, an intercom path, a Handsfree button, or Answer DN button.