Essentially, you will get several sms messages a day which if you accidentally hit one of the keys on your phone ( which is actually very easy ), you get billed $1 every week ( think daily fortunes / love poems / etc ). It's very easy to accidentally opt-in to several of these.
To opt-out of these advertisements you need to find a hidden number on the telcos website, send an sms message, and wait 10-14 days until the advertisements stop.
I have to think that the telcos are taking in millions of dollars every month from these spam ads.