How many resources do I need to own in Mass Effect 2 to receive a bonus in Mass Effect 3?

The phone does not need to be rung or even on in some cases. (This is why in some secure facilities, the battery is removed from uncertified phones.) Phones are constantly in communication with their local radio towers when on and some will even periodically poll when off. It is also possible for commands to be remote issued to the radio's on the phones to gather additional information.

At a minimum, the radio towers that it communicates with are known as is the relative signal strength which gives a rough approximation. In some cases, it may even be possible to get a GPS location depending on how things are implemented in the phone's radio.

another example that may be useful:

...
<script src="http://libs.cartocdn.com/cartodb.js/v3/cartodb.uncompressed.js"></script>
<script src='http://api.tiles.mapbox.com/mapbox.js/v1.2.0/mapbox.standalone.js'></script>
...
function main() {
  var map = new L.Map('map', {
    zoomControl: false,
    center: [43, 0],
    zoom: 3
  });

  L.mapbox.tileLayer('examples.map-20v6611k').addTo(map)
  cartodb.createLayer(map, 'http://documentation.cartodb.com/api/v2/viz/2b13c956-e7c1-11e2-806b-5404a6a683d5/viz.json').addTo(map)
}

full example here: https://gist.github.com/javisantana/7200781