Why is a call on the Smartphone lost in case I'm getting close to MobileDock?

As MobileDock is paired to the Smartphone acting as a Bluetooth handsfree set, the Smartphone transfers an ongoing call to MobileDock as soon it's in Bluetooth range (similar to a car handsfree set).
- Call can be taken back to the Smartphone
- Call can be taken and continued on ringing DECT Handsets
- By leaving the Bluetooth range of MobileDock the Call is transferred back to Smartphone