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.
Not sure what you're referring to... but yes, you need to buy a phone number. Head to ipkall.com (free) or icall.com or flowroute.com (paid). There are several such services out there. Once you have the number, you can just point it to your server that is running FreeSWITCH and Plivo.
The entire idea with Plivo (much like Twilio and Tropo) is to simplify i.e. let web developers build telephony apps without having to learn much about a telephony engine.
Twilio being open source - this is a HUGE misconception. Openvbx is just an application on top of Twilio and basically locks you in to the Twilio platform. That's the bitter truth. Plivo is a 100% open source version of what Twilio has been hiding from you.
As mentioned by akalsey, Tropo could be a good option if you wish to use a hosted service. If you wish to have greater control and host things at your end and use your own carriers and phone numbers, then Plivo is the way to go.