Mobile users on the O2 network have been experiencing problems on Friday morning.
There have been issues with making calls, accessing 4G systems and unable to use data.
The issues were reported just before 9am and outage recording website Down Detector showed that while the main issue was in the London area, customers in Cardiff were also experiencing problems.
"We're aware of an issue," the company's coverage status page reads.
"Our technical teams are investigating. We apologise for any inconvenience."
Some customers have been complaining as it has happened before:
It comes three months after the O2 network went down and customers were unable to use 4G.
Customers on Tesco Mobile and GiffGaff were also hit by that outage.
In a statement, O2 said: "Our technical teams are investigating reports of issues when some customers are making and receiving voice calls.
"Data use is not impacted. We apologise for any inconvenience."
Customers saw a rise in bills last year:
* If you have been affected by O2 problems, email cathy.owen@mediawales.co.uk.
