asterisk users - Apr 2007 - Change to Sip in a phonecall, when the user registers meanwhile

Hello Folks!

Is it following possible with Asterisk:

User1 is known to Asterisk with a Sip Phone and a Mobile Number.

If there is an ingoing phonecall for User1 Asterisk always tries to connect the
call to the Sip Phone.
BUT: if the Sip Phone is not registered, Asterisk calls the Mobile Number.

And know the complicated part:
If the user registers while Asterisk is ringing at the Mobile Number, Asterisk
should cancel the call to the Mobile and invite the Sip Phone.

How could I do this!?

Thank you very much for every help!


Kalle