As Irishhussar has already clarified there would be a prefix dependant on the Callsign Indicator (CI) on the daily changing BATCO sheet. There would also be suffixes too. So your 10, 20, 30, 40, 11, 21, 31, 41 etc etc, callsign could look something like this: B10, B10A, B10B, B10C, B10D, B10E, B20, B20A, B20B, B20C, B20D, B20E etc etc. This would be the format for all types of callsigns in this style. However your command callsigns 0, 0A, 0B, 0C, 0D. would not have suffixes or prefixes. Quote from wikipedia British ArmyTactical voice communications ("combat net radio") use a system of call signs of the form letter-digit-digit. Within a standard infantry battalion these characters represent companies, platoons and sections respectively, so that 3 Section, 1 Platoon of B Company might be F13. In addition, a suffix following the initial call sign can denote a specific individual or grouping within the designated call sign, so F13C would be the Charlie fire team. Unused suffixes can be used for other call signs that do not fall into the standard call sign matrix, for example the unused 33A call sign is used to refer to the Company Sergeant Major. Note that the letter part of the call sign is not the company's own letter (B vs F in the above example) - indeed, the letter designations are randomly assigned using BATCO sheets, and appear on CEIs (communication electronic instructions), and change along with the BATCO codes every 24 hours. This, together with frequency changes and voice procedure aimed at making every unit sound the same, introduces a degree of protection against simple traffic analysis and eavesdropping. Not all radio users fit into the standard battalion model, but in order to continue the obfuscation they will be assigned a call sign that appears to be part of such a system. Presumably, the well-known B20 falls into this category. Finally, the controller of each net has the call sign 0 ("zero"). There may also be a second controller - either a backup station or a commander who has delegated communication tasks to a signaller but may occasionally wish to speak in person - using the call sign 0A ("zero alpha").
