O2 and Three have confirmed issues with their mobile network and said they are investigating the issue.
In a message on its website, O2 said it was aware of a problem with its “voice services” and apologised to customers.
Three UK also confirmed an issue with voice calls is affecting “around 3%” of customers.
The fault comes as millions of people begin working from home as part of Government guidance to reduce social contact to help stop the spread of coronavirus.
According to network status monitoring website Down Detector, users of all four of the UK’s major networks have reported issues.
However, EE and Vodafone have not yet confirmed any faults on their networks.
In a statement on its website, O2 said it is looking into an issue.
“We are aware that some customers may be experiencing issues with our voice service. Our technical teams are investigating,” the company said.
“We apologise for any inconvenience. Updates will be shared here as soon as they are available.”
A spokesman for Three UK said: “We are aware of an issue affecting around 3% of voice calls.
“We apologise for any inconvenience and our team is working to fix this ASAP. The rest of the network is stable.”