AASTRA DT292 for MX-ONE User Guide EN - Manual (Page 92)

Installation
DT292 92
Attach the battery to the phone as shown in the figure below.
Connect the adapter to the charger and connect it to the
mains outlet as shown in the figure below.
Notes:
Place the DT292 in the charger and charge it for at least 4
hours before using it the first time.
Make sure the local mains voltage corresponds to the
voltage on the charger.
Do not connect the phone to the charger without a battery.