What type of phones do you have and how are the lines programmed? Multicall or keysystem or singleline?
I've never been sastified with Mitel's answer for this. Typically we program unique lines for 1st and second lines and then forward to the second line on a busy condition for external calls. You don't get caller id that way. It will show "From XXXX".
(So Mitel, if you are monitoring, how about a DCR for this?)
I think, (not sure and don't have time to test right now) if you program the second line as a multicall, then it will work.
Unfortunantly, I don't think there is a way to show CLID for second line while on a different line if they're not multicall.
Ralph