There is no step by step for this, you really need to be versed in what you're doing.
Some quick tips, you'll need a reliable softswitch. Something that's redundant and on a redundant network. Level 3 does SIP, so you'll want to contact them.
It's not going to be "low budget" for testing. This is something that requires a real business plan.
Hardware (servers, network)
Software (softswitch, billing, custom coding)
Misc (Level 3 contracts, or similar and how you're going to get access to DID's in the National market.)
Manpower (people to deploy and manage this infrastructure)
You can contract most of this out, but you really want a solid business plan before attempting something like this. Unless your goal is to fail as soon as possible.
I've also probably skipped over hundreds of important details.
You may want to reach out to iCall or somebody like them to see what's involved. There is a considerable amount of resources required to pull something like this off.