I am working with a SX-200 ICP CXi controller, and when I put calls on hold from a set, it is if they are on exclusive hold, that is, the set you put them on hold from is the only set that can pick up the call off of hold. When you press the hold key, the display says "ok select line". Not sure what it is asking for, but I want to be able to put the call on hold and then select the line from another phone to take the call off of hold.

You NEED to read the SS user guide appropriate for the SS or IP phone you are using! It IS on exclusive hold, unless you specified key system mode in programming. For each phone that should have access to hold to/from other phones.

FYI, "OK Select Line..." is telling you that you can now initiate a second call, having held the first call.

You may also want to simply transfer the call to the other phone, and then answer it there.

I would suggest hiring someone skilled in Mitel to program the system the way you want.