In-band signaling is a telecommunications method where control information, such as routing commands or dial tones, travels on the exact same frequency band or logical channel as the user data. Historically, telephone systems used audio tones to control switches on the same copper pair that carried the voice call. This approach left legacy networks vulnerable to phreaking, where users played audio tones to manipulate billing systems.
The old PBX still uses in-band signaling, so you can actually hear the routing tones if you listen closely during call setup.
Related terms
Out-of-band signaling
A telecommunications method where control signals travel on a different channel than the voice or data traffic.
Touch-tone dialing
The Bell System's trademarked implementation of dual-tone multi-frequency telephone signaling.
Blue box
A historic electronic toll-fraud device used to manipulate long-distance telephone switches.