TalkTalk has signed a deal with O2 that will see its MVNO use the operator's network in future instead of Vodafone's.
The move comes as TalkTalk prepares to embark on a 4G network rollout for its customers.
The firm said O2's network represented a "major step forward in our long term strategy to build a small cell and Fibre-To-The-Premise network".
The multi-year deal will allow TalkTalk to use O2's 2G, 3G and 4G services across the UK.
Furthermore, the company said the agreement means it could offer its four million customers a high-quality mobile service and enable it to build up its quad-play business over the medium term. Currently, TalkTalk has 350,000 customers using its MVNO or 9.5 per cent of its total base.
"We are pleased to be able to announce our commercial MVNO agreement with TalkTalk, which builds on our heritage of being the trusted provider for many notable MVNO, public and private sector organisations in the UK," said O2 chief executive Ronan Dunne.
"This recognition, as a leading source of essential connectivity, comes as a result of our continued and significant investment in our nationwide network. We welcome the start of what we hope will be a successful and prosperous relationship for both parties."
The move comes as rival firms bolster their quad-play efforts. Virgin Media is already well established in the field with mobile services offered through EE's network. BT and Sky are also separately thought to be in the advanced stages of rolling out mobile networks as part of a quad play deal.
Dido Harding, chief executive of TalkTalk said the deal was a "significant development for TalkTalk, building on our success in mobile to date".
"We are ideally placed to benefit from the market shift to quad-play, with a sizeable and growing number of customers already choosing to take mobile from us. We see real opportunities in both the near term and further out, to work with Telefnica UK to further accelerate our quad-play growth," she added.
