People working from home due to coronavirus are overloading the phone system.
Problems are being reported by mobile phone networks O2, Virgin, Tesco, EE, Three and GiffGaff.
Hundreds of complaints about services have been recorded by website DownDetector.
O2 said: "We are aware that some customers may be experiencing issues with our voice service.
"Our technical teams are investigating. We apologise for any inconvenience.
"Updates will be shared here as soon they are available."
One frustrated phone user tweeted: "Working from home becomes increasingly more difficult when your internet keeps dropping out! @virginmedia sort your life out.
"Seriously, some people trying to be professional."
One user tweeted: "Is EE down? I was in the middle of a call with my mother about half an hour ago, call cut and dropped, and now I can't get through to her.
"Just keep getting 'call failed'."
Another added: "Is it just me or are the phone lines down? I suddenly can't make or receive calls."
And one joked: "I'm not into conspiracy theory but with this coronavirus outbreak and now phones down, some s*** is going down."
