gistfile1.txt

PhoneStateListener phoneListener;
    private void setupSignalStrength() {
        final TelephonyManager manager = (TelephonyManager) getSystemService(Context.TELEPHONY_SERVICE);
        phoneListener = new PhoneStateListener() {
            @Override
            public void onSignalStrengthsChanged(SignalStrength signalStrength) {
                if (manager.getNetworkOperator().equals("")) {
                } else {
                    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                        // See https://github.com/AlstonLin/TheLearningLock/issues/54
                        Log.d("SIGNAL",""+signalStrength.getLevel());
                    } else {
                        // Just show the full icon
                        Log.d("SIGNAL",""+signalStrength.getGsmSignalStrength());
                    }
                }
            }

            @Override
            public void onCellInfoChanged(List<CellInfo> cellInfo) {
                super.onCellInfoChanged(cellInfo);

                for (CellInfo info : cellInfo) {

                    Log.d("SIGNAL",""+info.toString());

                }
            }
        };
        manager.listen(phoneListener, PhoneStateListener.LISTEN_SIGNAL_STRENGTHS);
    }

gistfile2.txt

Permissions required for Telephony Manager :

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
<uses-permission android:name="android.permission.CHANGE_NETWORK_STATE"/>