Well according to Ericsson, tens of millions…

 

On Thursday last week (December 6th 2018), millions of UK O2 users were unable to text, call or use their 4G connections.
Late that same evening the Swedish technology company Ericsson, which creates most of O2’s backend network admitted that the service downtime was due to an expired certificate! These certificates manage the way a service is provided to O2 customers and put simply, the software will not work if the certificate isn’t up to date or has expired!

This meant the certificate had to be reissued and updated which is also why users were kept offline for so long, with each Network node having to be updated manually, one by one. Ericsson CEO, Börje Ekholm stated:

 

"The faulty software that has caused these issues is being decommissioned and we apologize not only to our customers but also to their customers. We work hard to ensure that our customers can limit the impact and restore their services as soon as possible."

 

Most O2 customers were off line for almost 24 hours, causing chaos for all O2, Tesco Mobile, GiffGaff, Sky Mobile and Lyca customers, which all use the O2 network. O2 has since apologised and has given all customers credit for two days of their monthly airtime subscription.