You ring one of the australian sipbroker pstn access numbers (
http://www.sipbroker.com/sipbroker/action/pstnNumbers), dial *269nnnnnn where nnnnnn is your astratel number, and bingo, your phone rings.
This assumes your ATA is registered to astratel.
If it's registered to pennytel, then it would be *234xxxxxx where xxxxxxx is your pennytel number.
If that works, then you tell your OS person to ring the nearest PSTN number from the access number list, and go.
It cost your OS person the cost of the call to the access number. It costs you nothing.