Back in 2012, Ericsson was held responsible for 2 different O2 outages in about 4 months. O2 stated that they were removing the offending Ericsson equipment, and ending their relationship with Ericsson entirely. However, as the second link below shows, they simply replaced it with dodgy equipment from the same company.
So today we have another outage that has been apparently caused by the same company, Ericsson. But after many outages caused by Ericsson's equipment or software, and pledges to replace them with another supplier, the blame for this latest fiasco should lie squarely with O2, and they should not be allowed to pass the buck yet again.
Further reading reveals the problem was caused by an expired security certificate (these are used to certify software is genuine and hasn't been tampered with).
When a certificate expires a system considers it untrustworthy (which is why you sometimes get the problem with site certificate on a browser) and thus might well stop using the functions provided by the software.
It looks like someone forgot to renew the certificate at Ericsson
The delay may have been caused by engineers having to go to each piece of kit to renew the certificate manually or more likely the new certificate will have been obtained and 'seeded' by Ericsson who would have had to wait for the kit to 'phone home' to obtain the new one. (many devices in your home that are connected to the 'net 'phone home' to check for software updates every day or two days without you even knowing it).