A consumer-focused Sim card that automatically connects users to whichever UK mobile network has the best signal is set to go on sale.
Lancashire-based start-up Anywhere Sim aims to launch its notspot-tackling service within five weeks.
A small number of other mobile virtual network operators (MVNOs) already offer national roaming in the UK, but target their products at business customers.
One expert said demand for the new service might be limited by its cost.
The main operators - Vodafone, EE, O2 and Three - have resisted pressure from the government to offer national roaming themselves.
Business Secretary Sajid Javid had proposed that the firms automatically switch users to different 2G signals when their own networks dropped out.
But the operators instead favour extending their own coverage and creating bilateral network-sharing agreements to tackle the problem.
Anywhere Sim has managed to set up its service by pairing with a business based outside the UK, which already has roaming agreements with the country's networks.
"[It will appeal to] people who have experienced coverage issues," explained the firm's founder Matthew Wright.
"So, people who live in rural locations.
"And those who travel a lot and experience notspots - for example people that spend a lot of time in caravans and those who pursue outdoor pursuits - cyclers, runners, walkers, anglers, there's a long list that could benefit outside the big conurbations."
The firm will sell its pay-as-you-go Sims via its own website, eBay and Amazon. It also plans to team up with camping, farming and other organisations whose members might benefit from the facility.
The basic "home" service will charge 5p a minute for calls, 5p per SMS text and 5p per megabyte of data.
There are some caveats:
- Users cannot be switched between networks mid-call. Instead, if they encounter a notspot the call will end and only afterwards can they re-establish contact
- There is no 4G service offered as yet, although Anywhere Sim hopes to add one in about a year's time
- The basic service only lets consumers receive calls across all the networks, but limits dial-outs and data use to O2. Users will have to pay double the price per call and per megabyte of data if they want to be able to make calls and use the internet across all the networks
Even the basic service is significantly more expensive than many alternative MVNOs.
For example GiffGaff, which is limited to the O2 network, offers 500 minutes of calls, one gigabyte of data and unlimited texts for £10 a month.
To use up the same amount of talk and data time on Anywhere Sim's home deal would cost £75.
"You are paying slightly more, there is a small premium, but then you would expect that to be the case," said Mr Wright, adding that his service remained competitive since many users do not exhaust their monthly allowance.
He added that a pay monthly option was also being planned.
But one industry watcher remains sceptical about Anywhere Sim's appeal.
"There are always going to be some customers that will pay a premium for the best coverage but it's a niche market," commented Ben Wood from the telecoms consultancy CCS Insight.
"And the value will decrease over time as well because the network operators will keep extending their coverage.
"Consumers also have the alternative of buying a second phone or a dual-Sim handset for emergencies - obviously you lose the benefits of a single number but it might be a more cost-effective option."
Another challenge facing Anywhere Sim is that it may not have the national roaming consumer market to itself for long.
The telecoms firm 24 Seven has run a similar service targeted at business customers called Jump since late last year.
Its chief executive told the BBC it had already attracted a "four-figure" number of subscribers - including staff at an NHS Trust - but he now had his eyes on a wider market.
"[Jump's users are] from coverage-critical industries, including healthcare, utilities, vehicle tracking and recovery, asset tracking - that type of thing," he explained
"As it stands we are targeting businesses.
"But we will launch a consumer product after the summer - it won't be focused on the roaming Jump element, but that will be a premium add-on."