Use to use it a bit before network update was enabled on VF as a go-to test number :)

Looks like spark have gone full account inhouse customer only with the updated voice guy

/edit however under their contact page...

 

Billing and
world clock

 

0800 000 000

When calling from outside the Spark network I'm pretty sure the time got ditched maybe 3-4 years ago as it was being used as a clock by so many people at the expense of Telecom.