Hard to say. So far, I've only given out my phone number to online services like gmail (woo 2 factor authentication!) or banks, but that's because my email and bank accounts are more powerful than my phone number and because very few services ask for it. I think there's a chance I wouldn't give out my phone number, and I can't clearly feel whether that chance is larger or smaller than my reluctance to pay $5. (Modeling myself from over a year ago is tough.)
This also runs into the trouble that instead of getting resources from users, you're spending them on users- texting activation codes is cheap but not free.
The recent implementation of a -5 karma penalty for replying to comments that are at -3 or below has clearly met with some disagreement and controversy. See http://lesswrong.com/r/discussion/lw/eb9/meta_karma_for_last_30_days/7aon . However, at the same time, it seems that Eliezer's observation that trolling and related problems have over time gotten worse here may be correct. It may be that this an inevitable consequence of growth, but it may be that it can be handled or reduced with some solution or set of solutions. I'm starting this discussion thread for people to propose possible solutions. To minimize anchoring bias and related problems, I'm not going to include my ideas in this header but in a comment below. People should think about the problem before reading proposed solutions (again to minimize anchoring issues).