Hi, I can do the project as a person with a lot of experience on SIP programs specially on Linphone with several times of doing same project.
But for your project , there is an issue for iOS:
apple Store will not approve if you make any link to a third party site for buying credit. Only Registration can be made by the website and not the TopUp. Actually you can do it by informing user to buy credit directly trough your website ((( the thing that Skype is doing... Even skype has removed direct "BUY" button from iOS app))))
for Android it is easy.
for Blackberry , the new "BelleSIP" is not compatible and you will have problem. I can do the project , but I will not make promise for blackberry. you need to work only on Black Berry OS 10 which still no Linphone for it.
I will deliver blackberry to you too , but Linphone for Blackberry is not really interesting.
but for Android is ok.
--- iphone , ipad (without payment trough APP links, because App store denies)
--- Android everything
--- No BB promiss