Well if you're in the US and want phone numbers, there are several free providers such as ipkall.com and paid ones such as icall.com, flowroute.com. In case of the free providers, you won't even have to bear the cost of incoming calls. That's a solid advantage you'll have if you use Plivo. Also, there are hundreds of such number providers around. Just google for them. Once you have the number, you can just point it to your server that is running FreeSWITCH and Plivo.