A method of operating a communications system comprising: a communications station of the communications system: generating encrypted speech frames and one or more encryption synchronisation frames for transmission; associating each of the one or more encryption synchronisation frames with one or more corresponding encrypted speech frames of the encrypted speech frames; transmitting to system infrastructure of the communications system a set of frames comprising the encrypted speech frames and the one or more associated encryption synchronisation frames; and transmitting to the system infrastructure information indicating which one or more of the transmitted encrypted speech frames should be replaced by the system infrastructure with a corresponding encryption synchronisation frame of the one or more associated encryption synchronisation frames; and the system infrastructure of the communications system: receiving the set of frames from the communications station; receiving the information from the communications station; using the information to determine which of the encrypted speech frames should be replaced with a corresponding encryption synchronisation frame of the one or more associated encryption synchronisation frames, and replacing one or more of the encrypted speech frames with a corresponding encryption synchronisation frame of the one or more associated encryption synchronisation frames to produce a modified set of frames; and transmitting the modified set of frames to one or more TETRA communications stations. a communications station of the communications system: generating encrypted speech frames and one or more encryption synchronisation frames for transmission; associating each of the one or more encryption synchronisation frames with one or more corresponding encrypted speech frames of the encrypted speech frames; transmitting to system infrastructure of the communications system a set of frames comprising the encrypted speech frames and the one or more associated encryption synchronisation frames; and transmitting to the system infrastructure information indicating which one or more of the transmitted encrypted speech frames should be replaced by the system infrastructure with a corresponding encryption synchronisation frame of the one or more associated encryption synchronisation frames; and generating encrypted speech frames and one or more encryption synchronisation frames for transmission; associating each of the one or more encryption synchronisation frames with one or more corresponding encrypted speech frames of the encrypted speech frames; transmitting to system infrastructure of the communications system a set of frames comprising the encrypted speech frames and the one or more associated encryption synchronisation frames; and transmitting to the system infrastructure information indicating which one or more of the transmitted encrypted speech frames should be replaced by the system infrastructure with a corresponding encryption synchronisation frame of the one or more associated encryption synchronisation frames; and the system infrastructure of the communications system: receiving the set of frames from the communications station; receiving the information from the communications station; using the information to determine which of the encrypted speech frames should be replaced with a corresponding encryption synchronisation frame of the one or more associated encryption synchronisation frames, and replacing one or more of the encrypted speech frames with a corresponding encryption synchronisation frame of the one or more associated encryption synchronisation frames to produce a modified set of frames; and transmitting the modified set of frames to one or more TETRA communications stations. receiving the set of frames from the communications station; receiving the information from the communications station; using the information to determine which of the encrypted speech frames should be replaced with a corresponding encryption synchronisation frame of the one or more associated encryption synchronisation frames, and replacing one or more of the encrypted speech frames with a corresponding encryption synchronisation frame of the one or more associated encryption synchronisation frames to produce a modified set of frames; and transmitting the modified set of frames to one or more TETRA communications stations.