Perhaps his results might be helpful:
On a smartphone, you can. That's exactly the problem.
It's been in the news multiple times these past few weeks - an Android scam epidemic. The way it usually works is that a banner ad in an Android app links to a WAP site (indistinguishable from a regular web site for the average user), and there a single click is enough to start the subscription. It's a VERY shady business, and laws are currently being passed in order to fight these jackasses, but currently it's up to us as users and advertisers to work around the problem.
Even Admob has served these ads...
Some providers provide an opt out from any third party billing whatsoever, which also includes premium rate numbers and premium SMS... Not all providers have this option though, and it's something that the user has to actively request and/or activate via the provider's Hotline.