Hi - Check out Mojosells http://www.mojosells.com/
Their basic subscription is only $10 per month for unlimited calls to phone numbers in USA and Canada. I'm not sure if you can call outside these countries though.
You may also consider MagicJack but you will need to ship the device to your VA. This option may be a bit costly and is only advisable if you have established long-term commitment with the VA.
I just found Mojosells' basic account does not include "calling".
I am now using Sonetel.com and it works great. You can purchase an online number and upgrade to premium so you can make unlimited calls to US and Canada. You need to download a softphone / desktop phone app so you can dial from your computer.