"We're working urgently to fix this issue and will provide a further update as soon as possible," they said.
Other networks have seen spikes in reports of outages on Downdetector - but these are likely to be customers from networks unable to connect to EE or BT phone numbers.
Social media users have been posting about a loss of services of both EE and BT mobile and landline services.
BT Group has more than 30 million customers in the UK and made £20.4bn revenue in the last financial year.