At the start of WW1 the British cut the underwater cables coming out of Germany meaning they had no direct connections to any where. Germany sent communications via neutral countries. The British placed spies in the various countries where the underwater cables surfaced and were then able to intercept all the traffic worldwide. This did raise some issues because we were technically also spying on our Allies.

British Signals Intelligence began in 1914 when it was realised that interception of German radio traffic could allows the UK to anticipate German plans. A good history of GCHQ will give you details of how Sigint was used and developed during WW1 eg Behind the Enigma: the Authorised History of GCHC, Britain’s Secret Cyber-intelligence Agency by John Ferris.