There is a little known service available on mobile phones called a callback tone or ringback tone. It allows the customisation of what other people hear when they call you, instead of the normal ring ring tone. So people who use it tend to have their phones play selected music tracks to their callers until they answer their phones.

Now apart from the risk of people irritating the heck out of their friends, it strikes me as a potentially lucrative idea to let people opt-in to play ads as their ringback tones and as a reward get to make free calls or something. The service could offer even higher rewards to people who handpick ads that they think might be of interest to their friends.

Of course the chances of success for such a service will only be as good as the rewards people get. Users would have to feel that those rewards are worth annoying their friends for.

Another issue is likely to be in how to measure the effectiveness of such ads and on what basis to charge advertisers. Should they be charged based on how many people hear their ads and/or how long for? What feedback does the advertiser get on how effective their ads are? This is of course no different to display or TV advertising but puts it at a disadvantage relative to cost-per-click and cost-per-action ads.

So what do you think? Would you end your friendship with someone over a silly little ad?