TelephonyManager tm = (TelephonyManager)getSystemService(TELEPHONY_SERVICE);
tm.listen(mPhoneListener, PhoneStateListener.LISTEN_CALL_STATE);

// somewhere else
private PhoneStateListener mPhoneListener = new PhoneStateListener() {
    public void onCallStateChanged(int state, String incomingNumber) {
        try {
            switch (state) {
            case TelephonyManager.CALL_STATE_RINGING:
                // do something...
                break;

            case TelephonyManager.CALL_STATE_OFFHOOK:
                // do something...
                break;

            case TelephonyManager.CALL_STATE_IDLE:
                // do something...
                break;
            default:
                Log.d(TAG, "Unknown phone state=" + state);
            }
        } catch (RemoteException e) {}
    } 
};