The enlarged business, named VodafoneThree, is 51% owned by Vodafone, with the remaining 49% held by CK Hutchison, the Hong Kong-based parent company of Three.
It intends to invest £11 billion over the coming decade creating one of Europe's most advanced 5G mobile networks, including £1.3 billion in the first year.
The combined group additionally hopes to deliver £700 million of annual cost and capital expenditure synergies within five years.
The mega-merger was initially agreed two years ago and given the green light by the Competition & Markets Authority last December. However, competition regulators insisted that both firms invest billions in the UK's 5G network and cap prices on their lowest-cost mobile plans for three years.
Critics of the transaction, which reduced the number of UK mobile phone operators to three, have expressed concern that it will lead to poorer service and higher prices for customers.
Rob Lister, managing director of Lister Unified Communications, said: "This creates a real juggernaut with both bringing complementary strengths to the party. With the endless growing need for bandwidth to support our anytime, anywhere consumption of social media, entertainment and video-calling, alongside business apps etc, the investment in 5G networks and beyond is enormous, so this merger will allow them to compete once again with O2 and EE.
"It's good news for customers technology-wise but time will tell whether it will be commercially beneficial. More so than ever, it will be important to work with independent, specialist mobile dealers like ourselves to make sure you get the best deals and best levels of customer service."
Simon Gardner, Lister's sales manager, added: "Amidst these challenges, Lister Unified Communications stands out by offering a beacon of stability with options for all the major networks. For instance, at the moment, there are great options with EE which guarantees no annual price increases for the entire length of your Lister EE 24-month mobile contract. This commitment provides businesses with the predictability and financial certainty they need to plan effectively for the future."
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