O2 says it has restored full network service
O2 said it has restored full service to all customers after hundreds of thousands were affected by disruption.
O2 tweeted: "Our tests show that 2G and 3G services are now back for all affected customers. Let us know?", more than 24 hours after the network received the first reports of problems.
A spokesman said all users should restart their phones while those with smartphones who had turned off their 3G service could now reconnect.
The company offered no further word on compensation after earlier saying that it was "focusing on getting full service resumed for all of our customers".
Engineers worked through the night to fix the fault with one of the network's systems which meant some mobile phone numbers were not registering correctly.
They restored 2G access earlier and told customers with smartphones to turn off their 3G data to allow them to make calls and send text messages through the repaired network.
Many disgruntled customers took to Twitter to vent their frustration at still being unable to use their phones.
Tesco, which uses O2's masts for its Tesco Mobile network, confirmed that some of its customers were also affected.
Consumer group Which? urged anyone who lost service to log any costs they incurred to claim compensation.
"We want to see O2 offer compensation to all customers who have been hugely inconvenienced by this service blackout and have been paying for a service they cannot use," said the watchdog's executive director Richard Lloyd.
"People should also keep a log of any costs they incur to help with any claims for compensation."
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