Could be that your server system get's overheated ... be sure to get an alarm of that ... wherever you are!
Step 1: Unassemble the phone
I got this old Siemens S45 mobile phone. One of these phones where the battery got broken after 1/2 year. While testing with other batteries those small contacts broke and the device got unusable. It still worked. I decided to use the phone for alarmin purpuses on my small Linux server.
Disassembling is not an easy task with this type of phone. You have to find these small plastic "noses" on the left and right side. Best I did to open the case was to slib between the small crack using my fingernails ... some broke during this. But finally the case was open and the parts lay in front of me.
Step 2: Putting power on the device
Next step was to put power on the device. I took a standard usb cable and cut it apart.
The red and the black cable are the ones that gat the power. The usb port is capable of giving you 5V on 1/2 amperes. This is enough for the phone to operate. Feed the cable through the hole in the plastic cover and solder it to the old battery contacts.
Step 3: Attaching the power switch
While the phone works with the power of the usb cable there is one problem if power gets down. The phone does not power up automatically when power gets back. Therefore I attached some small wires to the power button. These wires are then connected to an analog switch (CD4066) that is controlled via the parallel printer port of the server.