The subscriber number (SN) is part of the telephone number and is defined in the ITU standard E.164
.decimal number whose number of digits depends on the length of the local network code (ONKz) and the country code
(LKZ). All three numbers may have a maximum of 15 digits, whereby the country code can have one to three digits and the remainder refers to the network code (NKZ) and the subscriber number. If the subscriber line has extensions, then the number for the extension is added to the subscriber number.