Depends on your provider. 0168 works for Telecom. If you're with somebody else simply dial the number like you would any other US number. Virtually every provider out there will support it but you will pay the usual calling price for the US.
The US is one of the few countries in the world that allows calling toll free numbers via hacks like this.
technicaljoe: Was there a reason for Telecom using 0168, feels rather unnatural.
Yes. Tollfree numbers are desiged to work within a country, not outside it. Calling tollfree numbers from outside the country they belong to doesn't occur with any other country in the world that I'm aware of (for example you can't call a NZ 0800 from outside NZ)
Because 1-800 was prolific at the time and a large number of US companies don't advertise local calling numbers Telecom launched the 0168 service in the mid 90's to allow these calls to be made. 0168 calls took a different interconnection route as they were required to be terminated within the US directly for them to work as they couldn't simply be terminated in the usual way due to the simple nature of how a toll free call works (somebody has to pay for those inbound minutes).
I guess Telecom have simply kept the 0168 code because that's something people know. Many other telcos will just interconnect the calls by dialling the number as a regular call as most upstream providers who terminate calls within the US will now allow 800/888 calls to be terminated.
Just dial it like any other international number, no need to dial any code in front of it. If from a landline it will cost the same per minute as an international call to the USA. Free if using Skype ;)