the good news is, you're not stupid
This behaviour is changed a bit with v3.
So far I've figured that this has to do with the common Headset behaviour. On a Headset when a call comes in you press the button to accept or decline a call. If you accept it, the headset sends out a "AT+CKPD=200" - a extended key - to the phone and it'll transfer the sound to the Headset automatically.
A problem is, that this command isn't accepted if it comes from the "Serial" connection - it has to be from the Headset connection.
The workaround I've found out yet is: Go to your "Local services" in your BT-Configuration and open the properties of your "My Headset" profile. Then go to the "Notification" tab and check the "Visual" notification.
Next time a call comes in, you'll get this Bubble tooltip called "Phone Monitor". If you tap / click it and you confirm the upcoming dialog with "Accept" the connection will be established and the audio is transfered to your phone automatically.
I'll take a deeper look into the spec. if I can find something more comfortable