with betamax and all names they market it under you have to look out for:
some services will not allow you to use a hardware sip device but you can only use their software.
some services will allow webcallback. You type your number and the number you want to call and you receive a call and get connected. Normally calling this way is not for free and has to be paid according their rate schedule.
Rates, free destinations can vary from week to week.
Don't expect real customer support if you have any problems. Betamax customersupport is famous for not answering mail.
There is a forum at
www.voipbuster.com/forum where you can ask questions.