It is one of the UK's biggest mobile networks, but EE is suffering a major network outage, leaving users unable to make or receive calls.
The service started experiencing issues around 08:00 BST, according to independent analyst DownDetector, which tracks worldwide network coverage.
While problems were reported across the whole country, users in London and the South East appear to be worst affected.
EE told MailOnline that the issue affects voice calls between some EE customers and non-EE numbers only.
Calls between EE customers are working as normal, and calls to 999 emergency services are not affected.
'Some of our customers are reporting problems when trying to make calls to some numbers this morning,' a spokesperson told MailOnline.
Angry users took to Twitter to vent their frustrations at the network's outage.
User @AbbiWilson88 tweeted: 'As per @EE let me down again. Customer service is shocking.
Commenting on EE's outage, Ru Bhikha, mobiles expert at uSwitch.com, said: 'EE customers affected by this major voice call outage will understandably be concerned and frustrated.
'While it's good news that messaging and data are still up and running, the lack of a projected timeframe for a fix makes this a disconcerting situation for a large proportion of EE's some 30 million customers.
Today's outage is not the first time this year that EE has experienced issues with its network.
In January, EE's mobile network suffered issues with its 4G calling, leaving users unable to make or receive calls on the network.
In April, EE suffered a major outage, with customers across the UK unable to access 3G and 4G internet services.
Users across the country reported issues connecting to both the 4G and the 3G networks in May.
This warning message is often shown when either switching mobile phone numbers, or when there is an issue connecting to the internet.
A spokesperson for EE said: 'We had a data issue affecting some internet services for some of our customers on 4G.
Also read: Business SA lashes state's public service