trixbox extension tweak

I want to create a feature where say I have extension 1000, and I want to only allow extension 1000 to speak on the phone for 400 minutes, regardless whether it's talking to another extension or calling out... 400 minutes period... I also want to create a sub 1000 extension, where it could be any number but linked to the 1000 and shared the same 400 minutes... I also want it to have the capabilities to shared(default) or set an amount for each... in other words or perspective that could possibly lead to the same result... have a field for each extension to set the minute can talk... or a drop down to select the extension it want to share it with...