Once you sign up for a SipSolutions account, all services that you purchase are to be pre-paid in advance. This includes calling, system extras and add-ons, equipment purchases and number porting. You will need to ensure that your account is always in credit to ensure there is no disruption to your calling service.
SipSolutions is a prepay service just like many other cloud services and subscriptions available today. There are no contracts for calling and no setup fees.