TDC said later on Thursday that its operations had returned to normal and it was now investigating the cause of the outage.
Trains and buses in parts of the country also suffered delays due to signaling issues, with chaos in stations and people stuck on trains, Danish media reported.
The Centre for Cyber Security, Denmark’s national IT security authority, and a branch of the Danish Defense Intelligence Service could not confirm if the two incidents were related.
TDC Net said Thursday evening it had implemented a fix that allowed customers to make calls, although with a reduced sound quality. The company urged customers needing to call 112, Europe's emergency number, to remove the SIM card from their phone before placing the call.