Jetstar does not, to best of my knowledge, have any grace period on bookings where they can be cancelled penalty free

You have already accepted a $50 voucher

USA is an exception area where DOT requires that there be provision for cancellation within 24 hours where it has juristiction - A domestic Australian flight is not in that category

To make a call to Jetstar customer service cheaply, perhaps use something like skype to call , but do be prepared that you will likely be politely told that the change fee applies. The normal number is +613 9645 5999 and costs AUD0.032 per minute