The UK’s mobile networks are experiencing problems with their services.
EE told the BBC it is something “that is affecting all operators and we are working closely to fix it”.
The problem is being blamed on “interconnect issues” stemming from one operator, O2.
“We don’t believe it is connected to the rise in home working [due to the coronavirus],” added EE.
O2 has posted on its website that some customers are experiencing issues with its voice service.
It added that the issues were limited to making and receiving calls on its 2G, 3G and 4G networks, while data and messaging services were not affected.
O2 also denied that the problem stemmed from its network, saying it was a “cross industry issue” – but did not provide further detail.
Vodafone said that it was a “short-lived problem” only affecting around 9% of voice calls on 3G networks.
“All operators are working together on the matter,” a spokesman told the BBC.
Downdetector, a website which monitors network problems, is reporting issues for all four operators in a range of locations, including Birmingham, London, Manchester and Glasgow.
In a statement to the BBC, O2 said: “We are aware that some customers may be experiencing issues with our voice service. Our technical teams are investigating and we apologise for any inconvenience.”