Two of the biggest mobile network providers in the UK, EE and O2 admitted to network issues on Monday evening as customers are unable to make calls to landline phones.
O2 issued a statement saying: "We monitor this site 24/7 and use the data from your check to tell us if there are any problems that we’re not already fixing.
"It’s rare that we’re not already on it, but we like to be sure."
EE said: "We are sorry about the inconvenience we are aware of the issue with the phone lines, our engineers are working on the issue and will resolve as soon as possible."
Customers took to social media to highlight the issue and said that they were unable to make calls to landlines.
The Telegraph cited spokesmen for the networks as saying that the issue did not affect calls to other mobile networks, text messages or data usage.
Both the companies have over 50 million customers and EE also operates Orange and T-Mobile brands in the UK.
O2 said that its fixed line partner BT was investigating the issue and added: "We are aware of an issue where some O2 customers may be experiencing difficulties making calls from their mobile to EE mobiles and to landline numbers.
"BT, our fixed line partner, have confirmed they are investigating a problem within their network and we apologise for any inconvenience."