This is a device that you connect to the corded phone line, and when a call comes in, it will flash a light to tell you the phone is ringing.
This can be good for someone hard of hearing, or if the phone ringer is quiet or far away. Can also be used if you have a baby and do not want to wake it from a loud phone ringer.
According to the Radio Shack catalog from the year 2000, the Fone Flasher was $14.99.
This was late in the product at this point as there is already a mini fone flasher, and a fone flasher 2, with many other phone accessories. I checked many Radio Shack catalogs and found the earliest one I saw with the fone flasher was 1985 for $17.95.
Sources:
https://worldradiohistory.com/Archive-Catalogs/Radio-Shack/Radio-Shack-2000.pdf (PDF Page 54 / Catalog page 50)
https://www.radioshackcatalogs.com/flipbook/1995_radioshack_catalog.html (1995 catalog, Page 20)
https://www.radioshackcatalogs.com/flipbook/1990_radioshack_catalog.html (1990 catalog, Page 88)
https://www.radioshackcatalogs.com/flipbook/1987_radioshack_catalog.html (1987 catalog, Page 152)
https://www.radioshackcatalogs.com/flipbook/1986_radioshack_catalog.html (1986 catalog, Page 80)
https://www.radioshackcatalogs.com/flipbook/1985_radioshack_catalog.html (1985 catalog, Page 90)