Customers on EE have been left unable to make calls as the company's 4G network faces an outage.
The network says it is working on a fix after some customers were unable to use their handsets.
On Twitter, EE said: "We're aware that a small number of customers are unable to make some calls over 4G. We're doing our best to fix this quickly.
"Please keep retrying, and we apologise for any inconvenience. WiFi calling, as well as text and data services, are not affected."
Customers were being advised to turn their phones off and on again when reports first started. Down Detector said it had reports of problems in Birmingham, Manchester, Liverpool, Leeds, Nottingham, Bristol and London.
In a further statement, an EE spokesman said: "Some of our customers have experienced intermittent problems making calls on 4G this morning. This issue has been resolved for the majority of those affected and normal service for all customers should resume shortly.
"Calls over 3G, data services and text messages have not been affected, so customers have been able to get online and use apps and messaging services as normal. We apologise to our customers for the inconvenience caused."
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Ru Bhikha, mobiles expert at uSwitch, said: "EE is the biggest network in the country so this outage has the potential to affect a huge number of people.
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