Android has it's NDK to handle native code. I was just wondering how deep you can go with low level programming on this platform - whether you can for example control what the phone transmits through the GSM/UMTS network or see the raw data that are received.
Is working with the embedded phone modem generally possible on this/other platforms within the scope of their APIs?
I presume that interfering with the lower communication layers (like adding compression to the voice data, change of encoding and so on) means playing with the firmware of the communication modules and is generally something that the phone manufacturers don't support officially.
Anyway is there something about this topics you can recommend to read or search for?
A public API package (it's not really rich in terms of your requirements): http://developer.android.com/reference/android/telephony/package-summary.html
About the low-level APIs (the Radio Interface Layer of Android), you can read here: http://www.netmite.com/android/mydroid/development/pdk/docs/telephony.html
About implementing (modifying) the RIL: http://www.netmite.com/android/mydroid/development/pdk/docs/telephony.html#androidTelephonyRILImplementing
In this case, however, you need to touch the Android source code, so for application development, it is not an option.