Abstract: A billing scheme for a telecommunications network is described in which a billing record for a call including at least a plurality of data bytes is generated in a switch (10), each billing record is duplicated, and check information is generated from each data byte of a billing record and associated with each said data byte to form a protected billing record. Each protected billing record of a duplicated pair is transmitted via two independent transmission lines to processing means (40, 50) in which the associated check information is generated byte-for byte from each received billing record and the generated check information is compared byte-for-byte with the transmitted check information in parity checking circuits (60, 61). If for any one byte of a billing record, the generated check information does not agree with the received check information, that billing record is discarded.