Hi
For the sport which you only want to be able to answer calls, the best and cheapest way would probably be to just transfer the parrot kit in your GTI over to your diesel...everything should just be plug and play..depending on how its been routed/mounted should be fairly straightforward, just take your time.
for the GTI, your options are either original VW bluetooth...which is quite expensive...or...FISCON bluetooth...i've got the fiscon basic fitted to mine (didnt want it showing up on the RNS) and its fantastic, you can use the steering controls etc and everything pops up on the full red dot FIS, also allows you to use A2DP so you can stream audio from your iphone through to your RNS, also a complete piece of piss to fit! loom sits inbetween the original wiring and the RNS, you route your mic and thats it after a bit of vagcom coding to enable the telephone although i didnt even have to do this for some strange reason...
good luck!