Hi Amanda

Shari gave you a great answer and some really good starting points.
I just wanted to add that it's also good if you're prepared with your own questions - and actually leading the conversation in that way (asking the questions and getting the client talking about what their pain points are, drawing them into a better understanding of how they can utilize your services) is a wonderful way to maintain your composure and stay confident throughout the call.
If your PC has given you any indication prior to the call about what they do, what industry they're in, where they feel their business is needing support, etc. - use that to your advantage and do your homework before the call whenever you're able to.
There will be some potential clients who get in touch and you won't have time to research, so having a few standard questions to ask during any consulation will be important too.
You're 'interviewing' the PC as much as they are you...
Best of luck!