Possibly the cost of 1 call could exceed your credit. It appears these numbers have a tariff and can charge anything up to $5 per minute or $30 for a single call.
Heya @Wildangel, hopefully the info below helps:
Optus made a business decision to exit the 'Premium Rate Telephone Industry' on June 1, 2000. This means that Optus customers cannot access 190X services using the Optus network. If a customer dials a 190X number, they will receive an invoice from an alternate service provider (usually Telstra), for the particular 190X call. Optus does not provide support for these 190X calls.