Facebook fighting spam with phone numbers?

I went to post on a buddy's wall, and saw this.

Are they seriously trying to protect against spam? I've been on Facebook for years now. Why is this happening to my account, now? If you don't want this to happen of course, you can give Facebook your phone number.

I'm not really sure how this would protect against a spammer, since spammers make a profession out of spamming, and they could easily gain access to a disposable phone.

This seems more like a scheme to gather phone numbers to sell to telemarketers. I don't put much trust in Facebook, since their record on privacy, regarding sharing information with other companies, is pretty abysmal. Selling your information is the only explanation I can think of for why they would do this, because I can't imagine that they actually care about fighting spam preemptively. I am guessing that their system already in place, the one that relies on people flagging messages as spam would be pretty effective, no?