In the US, unlike the UK, when someone receives a text/SMS message they are charged for it according to whatever plan they have set up. This charge is normally the same as for sending an SMS. In the US market this makes sense, as the same charge mechanism is true for voice calls on a cellphone - you pay by the minute (no per second billing here) for every call you receive, wrong numbers, answerphone calls, everything. It appears though that T-mobile is taking this a little too far - preventing people from turning off their text messaging service even if they don't want it and are getting charges from 'wrong number' text messages. Hmm.
As an side, this charge mechanism is one the of the reasons why the mobile marketing association over here is so keen on double opt in for text messaging campaigns. This double opt in, where you confirm with a customer that they really do want to pay $1.99 for a ringtone or whatever before you charge them. However some companies are starting to really push the boundaries of this by allowing for web opt in to services as follows; customer comes to a cool ringtone website, customer enters their cellphone number & operator and checks a box that agrees to $9.99 per month fee for 20 credits and then clicks 'send' to get a confirmation message to their phone telling them how to stop the service, but not requiring a from phone optin. This mechanism is fine for people who actually want the service, but the problem is that you can enter anyone's phone number without their permission and they won't know what is going on. Nasty.
At work we are currently getting a pile of support calls from people complaining about such charges from a number of companies - this is due to some database problem at a few of the operators who seem to think that we are running the campaigns (and a big waste of our time it is to, thanks guys!). The list of companies that we're getting the most support calls from are mostly from Da Da Mobile - their support number is 888-214.0381 by the way. What seems to make this worse is that DaDaMobile offer an affiliate scheme where you can have this form submission signup on your site and you get a kickback.. seems that some people may take that as an opportunity to drive some quick revenue by entering random phone numbers, kind of like the Google AdWord fakers.
I don't know how this situation will pan out, for our part we're trying to get these issues addressed with various people but not much movement is happening and a lot of the damage seems to already have been done.