Siemens website does not show A125 instead shows A120.
Search results for A125
If you know the type of battery try at
Siemens Gigaset Cordless Phone Batteries Series
or in ebay/amazon/alibaba
your Siemens PIN is the default written on the manual.
The orange router pin is appearing instead in the configuration page of the router (at times you have a sticker in the bottom of the router or on the back of it)
Regards