The telephone in inter-war Britain was an important tool in both the identification and categorisation of individual hearing loss. Between 1912 and 1981, the British Post Office had control over a nationalised telephone system. Linkage between telephony and hearing has long been noted by historians of sound and science, and Post Office engineers in the inter-war period had considerable expertise in both telecommunications and hearing assistive devices. This article first demonstrates how the inter-war Post Office categorised different kinds of hearing loss through standardizing the capacity of its users to engage effectively with the telephone, and secondly investigates how successful it was in doing so. By utilising the substantial but little used material held by BT Archives, we can trace the development of the Post Office’s ‘telephone for deaf subscribers’ and explore how it was used to manage and standardise the variability of hearing and hearing loss within the telephone system.