Thousands of O2 customers in the UK have reported problems making and taking phone calls.
Many logged the difficulty they had using the network via online sites that keep an eye on the status of the UK’s telecoms systems.
The problems were logged by customers all over the UK.
O2 said the initial problem had been fixed, but that customers may still have some “intermittent issues” when reconnecting to the network.
The volume of people trying to reconnect to the network continued to overload it at certain points, but it was expected that normal service would resume on Thursday evening, an O2 spokesman said.
At 18:00 BST O2 said the problems had been fixed and the service interruption had lasted about 40 minutes.
The spokesman added: “Some customers may continue to experience some intermittent issues when reconnecting to the network.
“We’re investigating what happened and apologise for any inconvenience this may have caused,” he said.
It is not yet clear what caused the problem but many people reported that it suddenly became impossible to make calls.
The first messages about issues were posted at about 16:00 BST with some saying that the system had stopped working in the middle of a conversation.
The status page on O2’s website gave few hints about what had gone wrong. The BBC checked a sample of postcodes across the UK and the site responded saying phone masts in those areas were not working.
It added that this could make the service intermittent.
On Twitter, some O2 users said customer service agents told people to turn their phones on and off a few times to see if that fixed the problem.
Some found that they could only call people who were not on the O2 network.