magnus,
Well a recommendation of what would be required is to use an external ringer. The ringer can be either Analog or SIP, whichever is most convenient to you; I prefer Analog myself. Then it depends on your software version on how you proceed, but for the most part you will setup the phone as a User to use Dynamic Extension. The new Analog or SIP device would be a secondary phone set to ring when the desk phone does.
Thanks,
TE