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.
You can check the latest status in your area on the EE website here