Status tones

Status tones are short tones that are generated and placed on the telephone line while a call is being initiated, while it is being conducted, and while it is being monitored by a third party to inform all on the line of an active or defined event.
Examples.
A user with call waiting service will hear double beeps on the line when another party is calling to their phone, but the recipient is actively engaged on the line. The call waiting tone is known as a "Feature" in some Telephone communities, and is billed by the Telephone Company as an Enhanced Service.
Other examples include: