Amendment of existing booking

I need to amend a booking I made, but because we paid with a debt card you need to go to the car hire place to amend the booking(you can't amend a booking online). First of all my problem is the guy that is using the car and that paid for the car is working. He can't just take off to go to tempest and amend his book. Second of all Capetown internal Tempest car hire does answer their phone. I phoned twice now. You hold on and on and then the phone gets dropped. You phone again and you get a voice mail. Now if no one is answering the phones how will they now there is message. I need for someone to contact me urgently so I can inform about the car.