Bluff my call.
its a program on the app store that lets you :
"bluff my call allows you to have any number appear on caller ID, change your voice, record you calls and more" free calls are limited to 2 minutes...
i used it a couple times on my blackberry curve. havent tried it on my android.
check to comments to see if it will work for your phone.
if you have a fully updated N1 it might not work, but i dont see coments about other phones.