You have two choices, either add it to your rental package for £7.77 and it's use is free, or you can use it without adding it to your rental for 36pence each time.
If you have the facility as a 'calling feature', you're charged £7.77 per quarter for unlimited usage. (You can also have 'unlimited ring back' as one of up to four calling features for £15.57 per quarter, or as one of up to nine calling features for £21.57 per quarter).
If, as most people do, you pay per use, you're charged 35.7p as long as the ring back is successful. Unsuccessful attempts are not charged.
Chuck's link goes to the finely detailed document used within the telecommunications industry. You might find the version for residential customers easier to use:
http://www.productsan...ternationalprices.pdf