Airtime distributor MoCo has been terminated by Three after it failed to meet connection targets set by the network.
MoCo had been set a target of at least 1,000 new business connections a month but fell well short, with barely 100 new connections this year.
MoCo will put nearly all dealer business connections through O2 going forward, alongside a number of direct connections to Vodafone.
MoCo sales director Adam Miller said: “It’s fair to say we’ve not done many new business connections with Three this year.
“We couldn’t live up to the obligations on us, and didn’t have enough in our armoury for it.
“But our focus has always been with O2 and that will continue to be the case. We have a 25-year relationship with it and, it is of far more strategic importance to us.”