Do you mean "off hook" in conversations, or numbers that are currently assigned to stations?
For off-hook I think I would be able to do it with DIS-SDSU in Expert mode by using CP or BUSY for Status, STNO for type, and PER2 for the level - I would have to mess with it.
I would use DIS-WABE with a type of GEN and specify the extension range if I was looking for numbers assigned to phones. The ones that are not in use have a status of "R" for Reserved.
I don't know how to do either in Assistant, sorry - sometimes I feel very limited by having a menu tree in my face, and other times like editing stations I like it better.