HI Chris,
There is no easy way to indicate in a SIP packet, which ringtone you want the phone to play (which annoys a lot of people because conventional switches can all play different ring tones for different kinds of call).
However, you CAN do it, with a little bit of tomfoolery and a SIP phone which supports multiple presences /lines.
Set up Identity1 on the phone as normal (normal extension etc etc.) Then set up identity2 at a different extension (you may have to configure this manually unles you absolutely understand the phone's provisioning parameters - make sure you DON'T put the phones mac address in the SAIL extension).
Now the phone has two numbers/extensions. In ecah identity, choose a different ring tone.
Set up two aliases for the phone/group of phones - one for internal and one for external (or whatever you want).
All done. You can now generate two different rings for each phone.
best
S