Phone network giants EE and O2 are suffering technical issues, with customers across the UK unable to make calls.
EE, the largest mobile network operator in the UK, is dealing with a barrage of complaints on twitter.
"We’re aware customers from EE and other networks are having issues calling landlines. We’re looking into why and we’ll keep you posted," the company said in tweet.
We’re aware customers from EE and other networks are having issues calling landlines. We’re looking into why and we’ll keep you posted.— EE (@EE) January 11, 2016
"We don't have a time frame but we expect it to be up and running again very soon."
EE said the problem was "currently only affecting calls from mobiles to landlines".
EE, which resulted from a merger between T-Mobile and Orange, has around 28m UK customers.
O2, the UK's second largest mobile telecoms provider, is also facing complaints.
A spokesperson for EE said:
Customers of a number of operators have experienced problems calling landlines for a short period. The issue has now been resolved.