The UK's mobile networks have experienced problems with their services.
EE told the BBC it was something "affecting all operators and we are working closely to fix it".
The problem has been blamed on "interconnect issues" between the operators.
"We don't believe it is connected to the rise in home working [due to the coronavirus]," added EE.
O2 had posted on its website that some customers were experiencing issues with its voice service but added that a full service was being restored. The alert has since been removed.
In a statement to the BBC, O2 said the problem meant that O2, Vodafone and Three customers were unable to connect to EE – and EE customers were unable to connect to O2, Vodafone and Three.
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, which had initially been blamed, saying it was a "cross-industry issue".
"At a time when the country needs connectivity most, it is important we work together rather than pointing fingers before facts have been determined," it said.
The firm added that a conference call had been scheduled with the communications regulator Ofcom to help determine the exact cause and "ensure this doesn't happen again".
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, had shown issues for all four operators in a range of locations, including Birmingham, London, Manchester and Glasgow.
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