2-tone paging
From JasonAntmanWiki
Two-tone (2-tone) paging is a format of audio-based radio paging commonly used to activate voice pagers used by volunteer emergency services personnel. The format is based on a list of possible audio tones, which are transmitted in a series of two tones to activate the paging receivers.
External Links
Two-Tone Paging on WikiPedia
Midians Manufacturer of paging encoders, also provides a wonderfully detailed wall chart of paging formats.