Sorry my misunderstanding, take a look here ...
http://www.voip-info.org/wiki/view/VOIP+Billing
The two sites you mentioned are both Betamax clones. You need to be speaking to a web designer who has done this work before. If you are able to write a detailed specification then you can put the project out to tender.
Alternatively have you considered a 'white label' Voip package? Someone else will handle all this side of the business, give you your own website and leave you free to do the marketing.