Calls not being routed to the Parrot Slim

The phone is a Nokia 6103. I'm able to pair the phone without a problem.

I can place a call using the Parrot and I hear it ring on the Parrot, but as soon as the other party answers, the audio gets transfered back to the phone's handset.

Same for incoming calls. The Parrot announces the call, but as soon as I hit the green button to answer, the call gets routed to the phone handset.

Any ideas?