I actually posted about this a while back here on HN, having figured this out when attending a course on SS7 signalling several years back. At the time i though

I actually posted about this a while back here on HN, having figured this out when attending a course on SS7 signalling several years back. At the time i thought it was a 'theoretical possibility' https://news.ycombinator.com/item?id=6682320 https://news.ycombinator.com/item?id=6682320