You can add funds to a user account and the user can then run campaigns within the available budget. Each outgoing call will check to ensure that sufficient funds are available. Learn more about ICTBroadcast
ICTBroadcast also offers REST-based API that allows billing and payments to be made from a web portal. So you don't need to worry about all the web portal related matters. ICTBroadcast has complete Service Provider features that makes it unique a solution in the telephony and broadcasting domain.