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.
It all depends on your budget... However if your looking at doing it properly with your own regulatory number ranges, interconnects, switching equipment. Your looking at a budget of atleast £40k/year.. Thats not including the buy rates.
Also you need to understand quality is everything in this market and most providers do not understand that... only go with regulator providers.... There are 10,000s of so called VoIP wholesalers (type wholesale voip) into google... you will soon see what i mean.... Now profit margins are wafer thin... Im being dead serious... I mean I will be open... In our company we make a loss if we include hardware, NOC, Tech Time, Admin Time on our wholesale dept... We only have it as we see it as an investment as more buying power and our corporate/public sector clients cover the loss made in the wholesale dept.
Most providers are just reselling another small provider... and for most people price is everything. However for our clients its not... We operate in a niche end of the market where quality is critical... Alot of our channel partners/resellers clients and multinational clients all perform mission critical services.
If you want serious quality and systems that are load balanced and most importantly a company that you can say is billing you correctly and are financially stable... Then you need to pick your upstream providers carefully. We have so many clients that cumulatively have lost over £20k simply as their provider went bust...
Also in this market there are lots of dirty and sneaky practices... one of the best one one is falsely answering a percentage of calls.. Most clients only learn until they have been burnt.
My honest word of advise is... unless you really want to do it as a proper business (not just resell, because at the end of the day... you are relying on someone else getting it right and usually your paying way over the odds for that system anyway) don't get into this market... Alot of people see it as a great market to get into... but realistically unless you haven't got the budget to implement a proper service, get the correct licences etc... your opening yourself to ALOT of legal cases... Remember. THIS IS NOT THE WEB HOSTING BUSINESS.
Also something we're seeing through our portfolio of clients are.. that there is not much money as people think in consumer VoIP as there is in Professional Corporate VoIP... But getting into this market is competitive and you do need your licenses, 24/7 NOC, SLAs etc etc.
If you want to run a business, then you need to be willing to put in the effort to figure out how to do that. Honestly, if you can't find this information yourself, you're in way over your head already.
A software you could look out which makes things semi-simplified is Asterisk. (Still not something that's extremely easy but it's a good start for you) TrixBox would probably be your best bet. It is an operating system that has Asterisk pre-installed for you.
I was looking at an Asterisk system myself for the home office in an effort to cut some costs, but the telephony cards can get extremely pricey. Especially if you want the noise cancelling mod added. I am still in the market myself for a cheap VOIP solution that will allow me to still use the phone while my Wife chats away to her friends.