Android: How do I get GSM signal strength for all available network operators

Maybe these quotes and links can help you code your own solution:

1.- To get a list of available network providers (quoting How to get a list of available network providers? in full):

Since Android is open source I had a look at the sources and finally
found something called INetworkQueryService. I guess you can do the
same as the android settings implementation and interact with this
service. Some guidance through NetworkSettings.java:

• onCreate starts the NetworkQueryService and binds it.
• loadNetworksList() tells the service to query for network operators.
• INetworkQueryServiceCallback is evaluated and if the event “EVENT_NETWORK_SCAN_COMPLETED” was raised, networksListLoaded will be
  called to iterate over the available Networks.

2.- Even a quick read to NetworkSetting.java and INetworkQueryService interface, gives us an idea to achieve your goal.

• Connect the service in declaration.

/**
 * Service connection code for the NetworkQueryService.
 * Handles the work of binding to a local object so that we can make
 * the appropriate service calls.
 */

/** Local service interface */
private INetworkQueryService mNetworkQueryService = null;

/** Service connection */
private final ServiceConnection mNetworkQueryServiceConnection = new ServiceConnection() {

    /** Handle the task of binding the local object to the service */
    public void onServiceConnected(ComponentName className, IBinder service) {
        if (DBG) log("connection created, binding local service.");
        mNetworkQueryService = ((NetworkQueryService.LocalBinder) service).getService();
        // as soon as it is bound, run a query.
        loadNetworksList();
    }

    /** Handle the task of cleaning up the local binding */
    public void onServiceDisconnected(ComponentName className) {
        if (DBG) log("connection disconnected, cleaning local binding.");
        mNetworkQueryService = null;
    }
};

• onCreate starts the NetworkQueryService and binds it.

Intent intent = new Intent(this, NetworkQueryService.class);
...
startService (intent);
bindService (new Intent(this, NetworkQueryService.class), mNetworkQueryServiceConnection,
                        Context.BIND_AUTO_CREATE);

• loadNetworksList() tells the service to query for network operators.

private void loadNetworksList() {
...    
// delegate query request to the service.
try {
    mNetworkQueryService.startNetworkQuery(mCallback);
} catch (RemoteException e) {
}

displayEmptyNetworkList(false); 
}

• INetworkQueryServiceCallback is evaluated:

/**
 * This implementation of INetworkQueryServiceCallback is used to receive
 * callback notifications from the network query service.
 */
private final INetworkQueryServiceCallback mCallback = new INetworkQueryServiceCallback.Stub() {

    /** place the message on the looper queue upon query completion. */
    public void onQueryComplete(List<OperatorInfo> networkInfoArray, int status) {
        if (DBG) log("notifying message loop of query completion.");
        Message msg = mHandler.obtainMessage(EVENT_NETWORK_SCAN_COMPLETED,
                status, 0, networkInfoArray);
        msg.sendToTarget();
    }
};

• If the event “EVENT_NETWORK_SCAN_COMPLETED” was raised, networksListLoaded will be called to iterate over the available Networks.

private void networksListLoaded(List<OperatorInfo> result, int status) {
    ...

    if (status != NetworkQueryService.QUERY_OK) {
        ...
        displayNetworkQueryFailed(status);
        displayEmptyNetworkList(true);
    } else {
        if (result != null){
            displayEmptyNetworkList(false);
            ...
        } else {
            displayEmptyNetworkList(true);
        }
    }
}

I hope it helps. I think it’s an interesting challenge so maybe I’ll give it a try next time I have some spare time. Good luck!