How often you do get new numbers. This is the big concern for me now before trying your service.
Our billing terms with most of our providers are monthly. Just under one month after the number was bought, we automatically release it. Next time a number of that type is needed, our system will purchase a fresh one. So the numbers exchange monthly. If you request a number type and we don't have any available (e.g. at busy times when they're all allocated) we also buy a fresh one. We also have a new service where you can request a pristine number, but it's a bit more expensive (see below).
Will you be ready to refund or provide me another number if the number I am going to rent, had got used for same type of service many times before and I will not be able to use for my turn.
Unfortunately, we can't offer refunds on our standard numbers in this situation. We have, though, just launched "Pristine numbers" which have not been used before (we buy them at the time you purchase them from us, rather than getting them from our pool). They are a bit pricey at the moment though (an extra 0.008 BTC). We're working on bringing that down.