A telecommunication method having the following steps: - establishing a speech channel for voice communication between a first and a second subscriber between a first telecommunication device of the first subscriber and a second telecommunication device of the second subscriber, wherein at least the second telecommunication device comprises a security module on which subscriber identification data of the second subscriber for establishing the speech channel and a secret key of the second subscriber are stored, and wherein the security module comprises a processor for generating a digital signature using said secret key; - transmitting data (138, 140) via the speech channel for authentication of the second subscriber to the first subscriber by means of a challenge-response process, the digital signature being generated by the processor in order to carry out the challenge-response process, and wherein the second subscriber is considered to be authenticated to the first subscriber when the challenge-response process is carried out successfully; - voice communication between the first subscriber and the second subscriber via the speech channel, wherein digitalised speech data to be transmitted from the second subscriber to the first subscriber via the speech channel is signed on a block basis by the second subscriber, wherein the signatures are transmitted from the second subscriber to the first subscriber, and wherein the first subscriber checks the signatures in order to ensure that the speech data has actually been received by the second subscriber, wherein the challenge-response process is carried out between the telecommunication devices of the first and second subscriber. - establishing a speech channel for voice communication between a first and a second subscriber between a first telecommunication device of the first subscriber and a second telecommunication device of the second subscriber, wherein at least the second telecommunication device comprises a security module on which subscriber identification data of the second subscriber for establishing the speech channel and a secret key of the second subscriber are stored, and wherein the security module comprises a processor for generating a digital signature using said secret key; - transmitting data (138, 140) via the speech channel for authentication of the second subscriber to the first subscriber by means of a challenge-response process, the digital signature being generated by the processor in order to carry out the challenge-response process, and wherein the second subscriber is considered to be authenticated to the first subscriber when the challenge-response process is carried out successfully; - voice communication between the first subscriber and the second subscriber via the speech channel, wherein digitalised speech data to be transmitted from the second subscriber to the first subscriber via the speech channel is signed on a block basis by the second subscriber, wherein the signatures are transmitted from the second subscriber to the first subscriber, and wherein the first subscriber checks the signatures in order to ensure that the speech data has actually been received by the second subscriber,