Just found out that Kogan mobile blocks outgoing calls to Rebtel local number.
This is Kogan's reply on the query:
The number you are trying to call (+61 8 6225 2803 / +61 8 6365 4442) is listed as "International-Premium" number and is one of the services excluded by Kogan mobile.
Are they allowed to do this? I can't find a list of numbers that they deem to be "International-Premium" on their website. Does that mean they're allowed to ban all local calling card numbers?
As I understand it, Rebtel is a service where you dial a local number (and are therefore charged a local call fee by your phone service provider (Kogan)), and you can then dial out to overseas numbers using Rebtel's network and you are charged by Rebtel for the overseas call.
A local call to Rebtel only costs Kogan the same as any other local call, so they do not need to charge you any more or block your calls to avoid any billing issues. However, if Kogan let you call overseas via Rebtel, you are unlikely to use Kogan for overseas calls, and that's why Kogan don't want you to do it - they want your money and don't want to make it easy for you to spend your money with someone else.
Are they allowed to do this? Yep. Basically, Kogan can make any rules for their service that they like, and your choice is to put up with those rules, or find another service provider - no-one is forcing you to use Kogan's services. Other providers will likely have similar rules, though.