Why do DECT Handsets start ringing when voice control ("Siri" or "Okay Google") is used?

MobileDock is paired to the Smartphone acting as a Bluetooth handsfree set. The voice control in Smartphone uses this connection to answer.

In order to avoid this, Bluetooth has to be unlinked in advance as follows:

Leave the Bluetooth range. (Connection status is show on Smartphone)
Switch off BT on my Smartphone
Push the on/off button on MobileDock. (all Smartphones will be unlinked!)