O2's data networks have been restored.
The company has confirmed that after a day of disruption, smartphone users are now able to use the mobile operator's services once again.
However, some consumers have complained they are still experiencing a few issues.
In response to the situation, an O2 spokeswoman said: "A small proportion of customers have reported issues sending text messages. We can see that the vast majority are being sent and received successfully and our technical teams are investigating.
"Our 3G and 4G data services are performing as normal."
Prior to that, O2 and mobile network equipment supplier Ericsson issued a joint statement about the problem, saying they were sorry for the disruption.
O2 boss Mark Evans said: "I want to let our customers know how sorry I am for the impact our network data issue has had on them, and reassure them that our teams, together with Ericsson, are doing everything we can.
"We fully appreciate it's been a poor experience and we are really sorry."
Meanwhile, Ericsson acknowledged that the disruption had been experienced in multiple countries.