OK, number one, your SX20 is long past it's prime. No new parts have been made for it since 1990! So yours was used when it was installed! All that is to suggest that it's time to replace it with a 50/EL/ICP. If you REALLY want to keep it going, be aware that SX20s have a bad habit of dying when turned off and then turned back on! Also, your memory support battery is probably long dead. Good, reliable replacement cards are few and far between. Do not go to E-bay, go to a reliable refurber like our sponsor here, or one of the other Mitel posters here that are in the business. Fifty5Footer is the one I remember, because it's a 'catchy' name.

Now as to the probable bad part, more likely one or more of the station cards is/are the culprit.

NOTE THAT YOU CANNOT REMOVE ANY CARD FROM THE SX20 WHILE IT IS TURNED ON AND RUNNING!!!! DOING SO WILL PROBABLY AT LEAST DAMAGE THE CARD(S) YOU REMOVE!!

P.S. I deleted your double post.

When I was young, I was Liberal. As I aged and wised up, I became Conservative. Now that I'm old, I have settled on Curmudgeon.