EE has been hit by a service outage across the UK, with customers reporting they are unable to make or receive phone calls.
The telecoms firm has responded to complaints on its Twitter account by telling users it was aware of the problem.
Users across the country have reported being unable to make or receive phone calls to some numbers from their mobile phones.
"Some customers are reporting problems when trying to make calls. We're working to fix this as quickly as possible," the company said on Twitter.
"All data and messaging services are working as normal. We apologise for any inconvenience caused."
The firm has not commented further on the problem, though some users have reported being unable to reach non-EE numbers and landlines.
Many customers had taken to social media to complain of issues with network service.
"I can't seem to make outgoing calls or ring your customer service from my landline - is service down? Any estimates on how long?", Twitter user Steph Plowright said.
According to network status monitoring website Down Detector, EE's network has been experiencing problems since early Tuesday morning, with some claiming their fixed internet connection had also been affected.