Possible Clues:

• One thing land-lines have that cellphones lack
• Cell phone's lack
• Hum on a line
• 'Go ahead and call' sound
• Caller's go-ahead
• Hum on the line
• Sound heard when a telephone is picked up (2 words)
• Open line indicator
• "Ready" signal of a kind
• It's a mix of 350 Hz and 440 Hz in the United States