the.com/dial tone

means The steady hum a landline makes to tell you the line is live and ready for you to dial a number.

from A compound of 'dial' — from the rotary dial phones of the early 1900s, itself from 'dial' meaning a marked disc (like a sundial's face) — plus 'tone,' the steady audio signal telephone exchanges generate to signal an open circuit. The phrase hardened into one term as rotary and then push-button phones made that waiting hum a universal, instantly recognizable cue; it later lived on as shorthand for 'the line is working' even as actual dials vanished.