Your best bet is to just get a Skype number and pay the fee to have a number and call anywhere in North America.
If you need multiple numbers I would go with a service such as evoice or grasshopper and and buy lots of them to forward that to your Skype number.
If you want to use specific techniques and see what converts best with certain phone numbers, I would use
http://www.callrail.com/
Unfortunately I couldn't get my affiliate account with them approved fast enough, but they are good to use! (probably a bit off topic of exactly what you are looking for.
