That's why I've just plugged the chargers in the car, put me on a tight-built street corner with medium bad reception, and set a DAB station without FM equivalent. As a load, I've plugged a half-unloaded smartphone to the store. For comparison, I have also tried the VW USB socket in the compartment of the control panel.
The results coincide with the measurements (Phew!):
Amazon:
The reception is usually okay, sometimes it gets "dull" (ie the error rate goes up, the usable bitrate goes down: error resilience, DAB is really cool.), But never stops completely. Plugging in your phone does not really make a difference.
Hama:
Inserted, reception bad. Mobile phone plugged in, reception gone. Immediate and reproducible. Nuff said.
RAVPower:
Even with plugged in cell phone no interference in the reception.
Voltcraft:
Bad reception, again and again dropouts.