Cell Phone Caller Tunes by Advertisers

Caller tunes for phones have been for quite some time. What I am wondering is, why cell phone companies are not using that for giving ads?

If we set any caller tune, we have to pay approximately Rs.30 per month. If cell phone companies put ads as the caller tune, and in return if they give some talk time to the person, then more people would put the ads as their caller tune. By that the advertisers get publicity, and the cell phone users get talk time in return, and it would benefit both.