THREE Mobile is down in parts of the country leaving customers unable to make calls.
At its peak, there were more than 900 reports on outage tracking website Downdetector.
Among the complaints, 82% related to customers struggling with their mobiles, 14% with phone signal and 4% with mobile internet.
The number of complaints has reduced throughout the morning but hundreds of customers are still experiencing problems with making calls.
The areas with the most reported problems include London, Birmingham, Edinburgh and Manchester.
One wrote: “Can’t make or recieve calls! What is going on? I use my phone for work purposes.”
Another wrote: “Is network down in Bolton apparently my number don’t exist.”
While one Twitter user said: “Unable to make any phone calls today on your network- what’s going on?”
A spokesperson for Three said they are aware of the problem and are working to fix it.
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They said: “It’s been brought to our attention that a small number of our customers are having issues making calls this morning.
“We’re working hard to get this resolved as quickly as possible.
“We understand this is far from ideal but please bear with us.”
We reached out to Three for more information on the outage.
A Three spokesperson said: “A number of customers may be experiencing issues with phone calls and we apologise to anyone who may be affected.
“The issue is being resolved and we hope to fix it fully in the next few hours.
“In the meantime, affected customers should restart their device which may fix it quicker.”
We will update this article when the issues are resolved.
It’s not the first time the network has seen problems.
Earlier this year, more than 3,000 customers reported being unable to text, make phone calls or access the internet due to a huge signal outage.
To check the service in your area see our handy guide.
Can I get compensation if I have no phone signal?
There is no automatic compensation scheme for mobile networks like there is for broadband and landline firms.
But that doesn’t necessarily mean you won’t get any compensation if your signal cuts out.
Previously, Three has given customers up to £20 off their bills following a nine-hour signal failure.
You should keep a note of how long the outage lasts for, and any inconvenience it causes you.
If you have to spend any money due to the outage, make sure you keep evidence of that, too.
To complain, you’ll need to call Three on 333 or follow its formal complaints procedure.