I retrofitted a Bluetooth module to my mk6 with a RCD510 radio. You need a bluetooth module that fits under the passenger seat, an additional wiring harness and a microphone to give you phone connectivity and voice control for dialing and talking. You need to be careful which Bluetooth module you get as not all are compatible with all mobile phones and not all can stream music. I think I found a useful table of part numbers and what each module does on the Ross tech website. You also need to code the changes for the module to be recognised. I think the total cost was about £100. The largest cost was the wiring harness at about £50.
The phone button only works with the Bluetooth module fitted and is used to pair your phone. My car now has the phone paired, recognises when I ask it to call a person using the microphone button on the steering wheel - eg call Joe, and the phone book is visible in the small screen and can be used to also select and make calls.
Later rcd 510 radios I believe had Bluetooth built in but not sure. The rns 510 does not.