Caller Abandonment
Refers to callers hanging up before reaching whom they intended to speak to. Usually occurs while the caller is on hold.

Captive Audience
Essentially, what your callers are while they are on hold. The majority of the time, those who call you are already interested in your company's offerings. That's why many businesses choose to promote their products or services in their on-hold message.

Endless Loop
Some tapes of on-hold messages are endless loop tapes, meaning they are played continuously until they wear out.