The feature you are looking for is called Camp-On. I think it is in the web admin online help, if not I don't have my work PC here, I can copy it for you tomorrow. Essentially you turn it on for the trunks, and they will Camp-On to an extension, with or without notification, and you can flash between the calls but it can make transferring a problem at times with single line sets. If you are using digital or IP phones, it is usually easier to make a Multicall appearance of the prime DN on the set.
