Order Centrex, bell system transfer, or whatever your LEC/CLEC might call it. Make sure you order this service for each phone line. Then add a recall button to your phone. When a call comes in, you can press recall, dial the destination number, when the receiving party answers the phone, press recall again and hang up. This is actually transferring off premise and not forwarding, but it sounded like what you were trying to accomplish.
Telecomboy was correct, R6 and higher for remote forward on the Partner ACS. Sometimes with POTS lines you will have db(volume) loss between called parties because of the line quality.
A.J. Wiesner
AT&D Communications