Your radio program being broadcast via phone can be picked up wherever the assigned phone number can be dialed.
Since a phone number in the U.S.A. is being assigned, it can be called from within the U.S.A. at cheap rates or even for free when it is in available minutes you have in your package.
When dialing from outside of the U.S.A. the rates might be higher, but there exist many ways to call a U.S. number at very affordable rates.
Alyx & Yeyi can add phone numbers in other countries depending on the popularity of your radio via phone service in the U.S.A.