thanks for the reply. I just found the right place.
The files are saved under "/data/data/com.android.phone/files/am/greeting/"
The files are saved as Adaptive Multi-Rate (*.amr)
i did it the following way:
1) recorded an audio with approximately the same duration as my audio file i want to use.
2) downloaded the audio file to my pc
3) opened the audio file i want to use on my phone in audacity and made it the exact same length as my recorded audio.
4) saved my modded audio file as amr and saved it on the phone. (placed in the same directory and using the same name as the previously recorded audio)
I am not sure if the stuff with the same length is really needed, but the first time i tried it showed 0 seconds on the phone and didn't looked like it works. (but it might just be that i had the sound muted or something.