Mobile operator O2 has said it will compensate its customers following a day of disruption to its data networks.
Customers with a monthly subscription will be refunded the cost of two days' service by the end of January.
Pay As You Go customers will get 10% extra when they top up their phone in the new year or 10% off when they buy data for mobile broadband devices.
On Thursday, O2 issued a joint apology with mobile network equipment supplier Ericsson for the shutdown.
The compensation for customers on monthly subscriptions includes small businesses and mobile broadband users.
Business customers are advised to contact their O2 corporate account manager to discuss compensation.
O2 has 25 million users and also provides services for the Sky, Tesco, Giffgaff and Lycamobile networks, which have another seven million users.
Services such as bus timetable information were also affected, while many businesses faced disruption.
How were people affected?
Luke Stagg runs a plumbing business and depends on his phone, but could not get through to customers or use his sat nav.
He said he would never know how many jobs he had missed out on during Thursday. In addition, he could not contact those he was supposed to be visiting, and his day was wasted.
"I'm 36 years old, so I know how to use a map, but I could not make calls," he said. "Mine was not a proper emergency but there is a wider point - I want to know what happens if it went down for even longer."
He said when he lost data coverage on a previous occasion, he was offered vouchers to use in the O2 shop. However, he was not impressed with the offer.
Mischa Bittar, also a plumber, said he had been "unable to contact any of my engineers or customers via email, unable to use our mobile systems to contact any engineers at all, so everyone's just had to down tools".
"A lot of money lost and the first thing I know about it is via the BBC website, no contact from O2 at all - disappointing," he added.
Omeran Amirat said he had lost bids for presents he was supposed to be buying for his children on internet auction site eBay. However, it is unlikely that he will receive compensation from O2 for this.
What caused the problem?
Ericsson president Börje Ekholm said "an initial root cause analysis" had indicated that the "main issue was an expired certificate in the software versions installed with these customers".
The company was carrying out "a complete and comprehensive root cause analysis", he added.
O2 said voice calls were not affected by the problem, but some customers said they could not make calls or send texts either.
O2 is owned by Spain's Telefonica and has the UK's second-largest mobile network after EE, which is part of BT. It is the company that bills customers, so it holds the responsibility for compensation.
Mr French, of Which?, said the company should "do the right thing" for customers, with the option of offering additional perks alongside refunds.
Are there any other considerations for customers
Many people would have used public-access wi-fi if they were unable to access data through the O2 network.
Mr French said this created additional risks if people were exposing personal information on these networks.