Make it easy and fun to use....
You can make the features for customized ring back playing available to your subscribers via various user interfaces. Ideally, you want to use the most popular method in your market, which is often the same method your subscriber uses to purchase ringtones, music downloads and wallpapers.
Obviously people without access to internet will not be so eager to use the web portal. In North America, people have a preference towards WAP and WAP push messages instead of SMS. In Africa, on the other hand, people tend to use the IVR. Use what works best in your market!
The personal circumstances of each subscriber may also play a role. Even those preferring the web portal over other alternatives may find it practical to buy via SMS if they see a ring back tone advertisement in a restaurant. Those without flat free mobile internet may be a bit more selective on when to use WAP.