AASTRA DT292 for BusinessPhone User Guide EN - Manual (Page 102)

Installation
DT292 102
Changing the battery
Rechargeable batteries have a limited lifespan. If the standby time
for the Cordless phone becomes too low, you should replace the
battery. Please contact your system administrator or supplier for
more information.
Note: Only use the specified batteries (NiMH pack 600
mAh, 3.6 V).
Do not disconnect or exchange the battery unless it must
be replaced. Do not charge the battery outside the phone.
The battery does not contain cadmium and is therefore more
environmentally-friendly than other rechargeable batteries.
However, all batteries should be treated as chemical waste.