According to outlets, the reports about malfunctions became more frequent starting from 14:00 GMT.
Later on Thursday, Hamburg police said emergency phone calls may not be available because of network failures. The police responded by increasing the number of officers patrolling the streets of the city.
"Interference with emergency 110 calls continues to occur. Therefore, our presence on the streets is increasing significantly. If necessary, contact a patrol car or go to the police station," the police said on Twitter.
German provider О2 confirmed the information about the disruptions.
"There are currently possible restrictions on calls on our network. This affects calls on 2G and 4G mobile networks as well as on the landline network," the provider said on Twitter.
At the moment works are being carried out to restore the network functionality, the company added.
О2 informed on Thursday that it solved the issue of network disruptions in Germany, adding that almost all customers can make calls again.
"The problem has been almost completely solved. With a few exceptions, our customers can make all their calls again. We are still working on a final solution. We are very sorry for the inconvenience caused," the provider said in a statement.
The provider noted a failure in the work of a server that transmits phone calls caused the problem.