O2 has said that 10% of its UK customers - over 2 million people - are having problems today when making calls or using mobile data.
The as-yet unexplained problem is expected to be fixed this afternoon.
Customers suffered around 24 hours of disruption in July when a software fault prevented a large number of people from connecting to the O2 UK network.
O2 says the problem was caused by the failure of one of its network nodes; a different issue to July’s disruption. A solution was found within three hours and engineers were able to start reconnecting customers from 3.30pm on Friday. Customers affected won’t be offered compensation for this outage.