The service provider is not allowed to ask for a manual or electronic signature from you (this is known as "written form"). If the service provider anyway does so in its AGB, then this clause is invalid.
If the service provider has not specifically regulated the form of termination, you can theoretically terminate the contract over the telephone. But we advise against this!
Because in case of doubt, you have to prove that the service provider has received your termination notice. It is best to send the termination notice by registered mail (Insertion in letter box (Einwurf) or return receipt (Rückschein). You are also on the safe side if the service provider acknowledges the termination notice in writing.
In which cases can I terminate the contract without notice?
There are only a few exceptions where you can terminate a telephone or Internet contract before the end of its term. You will need an important reason for such a termination without notice. This can be, for example, if the telephone or Internet service is permanently not working or is working very poorly ("non-performance" by the service provider or “consistently poor performance"). Here we explain when exactly you can terminate a contract in the event of disruptions.
Is a long term contract not for you? There are some Internet and telephone contracts that you can terminate any month or after one year. For mobile phones, there are prepaid contracts without any term.