I think the number provided by SA is just to limit any liability they may have. They want to ensure everyone (even the least tech savvy individual) has access to a discreet private number. It’s just an option. But legally they aren’t tracking your number. I read their terms of service with a fine tooth comb (and everyone else should too after the Ashley Madison breech). They track your IP address and activity on their site, but they can’t and don’t track anything once you leave their site.