On Friday, some users of Sky Mobile and Giffgaff reported being unable to place or receive calls or send texts.
The issues would be fixed "as soon as possible," according to a spokesperson for Sky Mobile.
Giffgaff acknowledged that problems with calls and texts had been reported by customers since "late morning.".
According to a Giffgaff representative speaking to the BBC, "Our technical teams are still working hard to fully restore these services.".
Around a thousand reports of issues with Sky and Giffgaff have each been made to Downdetector today. On the website, users voluntarily report technical problems.
Both networks are dependent on O2's mobile network, as is Virgin Mobile, which has just started transferring its customers to O2 networks in anticipation of the two operators' 2021 merger.
Also using the O2 network is Tesco Mobile.
Customers of O2's customer service were informed via Twitter that "technical teams are aware and are working to fix this as quickly as possible.".
The mobile phone outage occurs on a Friday, when many customers will be making plans for the weekend, even though Sky Mobile's data services continue to function.
Leslie Jones, a Twitter user, stated: "We have been without a signal for more than 24 hours. cannot place or receive calls. We have a 97-year-old mother, so kindly let us know what you plan to do to fix this immediately.
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