I have a mitel sx50 that has two trunks on it, it was working fine for a week and now it will ring twice when calling into the main line and then go to a busy signal, the console will continue to ring but when answered its a dead line.

Nothing is showing on the display, when the call comes in if you get it on the first ring you can answer it.

Any ideas?

Check lines with system disconnected using butt set. Check for foreign devices on the line, like ALARM SYSTEMS that answer after 2 rings after a call that rang 6-10 times before being answered. Check for defective c.o. line protection. Check for system transferring call to port that is/was for AA that no longer works/removed. And last but not least, the trunk card!

Disconnect a line from system and dial in. If same result you know who to blame. Is that GS or LS trunks?

LS Trunks, I will disconnect one of the lines ans see what happens. Then troubleshoot from there.

Make sure that the polarity is correct on the trunks too.