Yeah, I was glancing at this without really reading it: https://github.com/mozilla-b2g/gaia/blob/master/apps/callscreen/js/call_screen.js#L524

When you call the mock L10n library, it'll still return a string with numbers, just not the actually localized one. For example, if you pass in this:
```js
MockL10n.get('test', {m: 3});
```
It'll return:
```js
test{"m":3}
```
Sorry if I wasn't clear at first.