Possible Clues:

• Signal to make a call (2 words)
• It's a mix of 350 Hz and 440 Hz in the United States
• Landline sound
• Cease to be receiving singer with new noise on the line from America
• Sound on phone line
• Flexible on detail needed to make calls ...
• Feature of a landline, but not a cellphone
• Cellphone's lack (2 words)
• Sound around a cradle
• Sound on a landline phone