Thousands of O2 customers are reporting problems making and taking calls.
Many have reported difficulties using the network via online sites that keep an eye on the status of the UK’s telecoms systems.
The problems seem to be widespread with reports being logged by customers all over the UK.
In a tweet, O2 said it was aware of the issues and had engineers looking into the cause as a “priority”.
It is not yet clear what has caused the problem but many people reported that it was impossible to make calls.
The first messages about issues were posted around 16:00BST 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 post codes 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.