Problems began around midday and have continued into the evening, impacting areas including the Central Belt of Scotland.
The firm tweeted: “We are sorry that a number of our customers are having trouble using our mobile services.
"We’re working to get things back to normal as quickly as possible and will keep this thread updated.”
A spokesman said EE broadband and landline services were not affected and customers could use Wi-Fi calling.
The Daily Mail reported more than 900 problems had been logged on Down Detector, with Edinburgh and Glasgow among the worst hit areas.
You can check the latest status in your area on the EE website here
