Tesco Mobile seems to have crashed in parts of the country leaving thousands unable to make calls.
Close to 4,000 people reported issued with the network on independent outage monitor Down Detector.
The true number of those impacted is likely to be far greater.
Reports have been made from across the UK, from the south west of England all the way up to Aberdeen in Scotland.
Aggrieved customers took to Twitter to vent their frustrations.
Jessica Knowles wrote: "@tescomobile are you having difficulties? I can't make phone calls and neither can my mother who is also on Tesco mobile."
Advice offered to those complaining has been to restart your phone and clean your sim-card.
The suggestion did not prove immediately successful for everyone.
Charlotte Potts wrote: "Four of us on tesco mobile in my house none of the phones ringing out?
"When will this be resolved please? Re booting and cleaning sim doesnt work."
Richard, who is working Tesco Mobile's social media account, assured concerned customers that the issue was being looked at.
He wrote: "Don't worry we're onto it. We've got engineers investigating so we can get it sorted for you ASAP."
More to follow.