Mobile network operator O2 claims it has fixed network problems that left hundreds of thousands of customers without voice and data connections for up to 24 hours.
The firm has issued a string of apologies, but has not yet made any statement about how it will compensate customers.
Those affected by the network problems included subscribers to GiffGaff and Tesco Mobile, which use O2's mobile infrastructure.
"Our tests now show that all our 2G and 3G services have been fully restored for affected customers. If any customers are still having problems we recommend they turn their phone off and on again," O2 said in a statement.
The company is yet to reveal the exact nature of the technical fault, which caused the network to fail from 1.30pm on Wednesday. O2 revealed only that it had narrowed the fault down to the part of its network that deals with registering numbers.
In a statement on its blog, O2 said there was "a fault with one of our network systems which meant that some mobile phone numbers were unable to register correctly on our network".
Cause of O2 downtime a mystery
Two months ago, O2 confirmed a five-year contract to hand over the planning and management of its network to Chinese firm Huawei, leaving questions as to whether the migration was a success.
Neither Huawei nor O2 would confirm to Computer Weekly what hardware had faltered, but the equipment is not provided by Huawei, raising questions about whether technology provider Cisco was to blame.
However, three hours after Computer Weekly contacted the networking firm, Cisco said it had spoken to O2 and confirmed none of its equipment was at fault.
Some industry analysts have speculated that the problems were caused by a software failure rather than anything to do with hardware
The BBC quoted Steven Hartley, principal analyst at Ovum, as saying the fault may have been related to the Home Location Register, which is used to check that someone is a legitimate customer. This ensures that unauthorised SIM cards are not able to use the network.
The process also generates the encryption key used to ensure nobody can intercept subsequent calls, texts and data transmissions.
This theory is supported by the fact that some O2 customers were able to connect to the network, while others in the same place were not.
After services had been restored, O2 said in a blog post that it remained "focused on identifying the root cause of the incident".
