Not sure if it will work for a ringing trunk but check out the Inquire button, it allows you to view CID of a call and have the option of answering it or not. It may only work for parked or held calls though.
Nameless:
The ISPY button is an investigate button, similar to a barge in that it allows you to see who a station is talking to and view the number dialed or incoming CLI then you are given the option to barge in.