Hundreds of O2 and Vodafone customers have reported problems with the networks.
The reason for the outages is unclear, but, according to Downdetector, the issues started at around 12.09pm, and are affecting O2 and Vodafone customers across the UK.
More than 5,000 O2 customers have complained their signal has dropped and they can't connect to the internet.
O2 said it’s aware of the issues, and its technical teams are investigating.
It said: “We’re aware of an issue impacting some voice calls. Our technical teams are investigating.
"We apologise for any inconvenience. Updates will be shared here as soon they are available.”
Hundreds of Vodafone customers also reported problems on Tuesday afternoon.
But a Vodafone spokesman said "there are no issues with the network".
"Some of our customers may have had difficulty reaching users of another network, which reportedly had a problem," he said.
Later on Tuesday, an O2 spokeswoman confirmed the issue with voice calls had been resolved.
"Our technical teams are continuing to monitor the situation," she said. "We apologise for any inconvenience."
