Hi Michael,
My wife used to work as a virtual assistant and clients used to pay a flat rate per hour; say 3 hours per day at 12.00 per hour. During this time she answered phones, emails, do research, an anything else the client wanted.
I m not sure what you call volume is but if you do it per minute it may get expensive as the VA could drag a conversation for a long time and charge accordingly. Furthermore, and it would be a nightmare for you to track the usage.
I m not sure about oDesk but my understanding is used mainly for one time projects. Have you looked into Elance?or BrickWork?
And depending on your budget, you could hired someone for one or 2 hours per day. The rest of the time you could manage calls using Google Voice. This service is free and when someone calls google transcripts the voice mail and email or text it to you. This way you can be on top of it at all times.
One last note about VA, make sure you give them a transcript of what to say in your behalf. They need to represent you in a professional manner.
good luck.