Ignore my answer above with suggested reading... I must have been reading two different things at once, had a few too many when I read the question, or not enough coffee, I am sure one of those situations applied because somehow I thought you wanted to do "whisper announce" which is basically Off-hook Voice Announce (you can make a "page" to a phone that is on a call).
Ignore what I said before... :/
And the answer is no, you can't do it in the phone... BUT you can adjust transmit and receive levels with some headsets, like many Plantronics units but it often distorts the audio or causes echo if you tweak it too much, so it's a 50/50 shot if it will work in your case.