asterisk users - Mar 2006 - Aastra 480i CT - multiple lines?

I just set up an Aastra 480i CT with separate registrations on my Asterisk
server. The way I set it up is Line 1 on the phone is registered to 101 on
the server and Line 3 is registered to 103.

If Line 1 is being used and a call comes in on 101, it rings to Line 2. But,
if Line 3 is being used and a call comes in on 103, the phone responds
"Busy
Here".

Is there any way of assigning groups of Line buttons to different
registrations (like the snom phones)?

Nabeel