VTech VCS601 Guide D'utilisation page 18

To charge an external device using the speakerphone battery:
1. Plug the USB cable in the USB type-A port of the speakerphone.
2. Plug the other end of the USB cable in the USB type-C port of the external
device.
NOTE: The speakerphone must be on and the battery must not be low.
You can charge the battery of your speakerphone (USB type-C port) and
charge an external device using the speakerphone battery (USB type-A port) at
the same time.
18