As a freelance Virtual Assistant, if you need your clients authorisation to speak on their behalf to a company i.e. changing a flight or cancelling an account one thing to consider is dialing the client into the call so they can authorise you to speak on their behalf.
Skype provides an easy way to do 3 ways calls - simply make sure that you've made the original call using your Skype account (don't forget to keep your Skype account topped up with funds) and then when the time comes to receive authorisation you can dial the clients number so they're part of the 3 way call.