on-hold advertising
n. Telephone-based advertising directed at consumers while they are waiting on hold.
Many hvacr contractors are turning to telephone "on-hold" advertising as an effective, inexpensive way to market products and services to a captive audience.

Listeners on the other end of the line will hear about spring clean-and-check specials, financing on a new air conditioning system, or air cleaners to help them with their allergies.
—John R. Hall, “Commercials on hold are music to the ears of hvacr contractors,” Air Conditioning, May 31, 1999
1989 (earliest)
Admittedly, there is little research on how effective on-hold advertising is, said Stagliano. He cited a study that found 88 percent of callers who heard music or messages on hold liked the system, while 16 to 20 percent who heard product information actually bought the product.
—Lynette Hazelton, “The Hold Co. Holds Onto High Standards,” Philadelphia Business Journal, April 03, 1989
In the past, I also provided start-up assistance for two other businesses: A Montessori school for pre-school children in Michigan and more recently, a company called, On-Hold advertising in Tennessee.
—“New economic realities: the role of women entrepreneurs,” United States Congress House Committee on Small Business, January 01, 1988