I would think having a world wide directory would be unnecessary. If ppl wanted to receive SMS messages from your site, I would think a few things would be crucial.
1-Registration of the person trying to receive SMS
2-The person sending the SMS should know how to do that on their end ie have the address.
I know that I dont need a global directory to send emails and SMS's to my friends.
Databases of this nature have lead to poor usuage of bandwidth. Creating senquential numbers behind specified prefixes is the potion for SMS spam. If the carries intended their users SMS addies to be public, like the white pages, they would be freely published.
I would assume the carriers would frown upon anyone compiling such a list and possible take court action for illegimet use of the SMS system.
Therefore, I would be very careful how you present this information. You leave yourself open to liability and possibly being charged the carriers rates in reverse for using their service in an unauthorized manner.
I would think the first course of action is to goto the specific carriers and respectfully inquire about what impacts your application may have. They may also freely give you the information and then you can act in a lawful manner.
Finally, the end user may not use your information as honorably as you might and unwhittingly put you at legal risks.
Respectfully I would advise that you be mindful in your construction of your application should you come across the data that you seek.
Happy Hunting,,,,