What are the acoustic cups for on TTY ?

If you're talking about a device for the deaf, when they first came out, they were hugely expensive. You carried them from place to place and most of the phones were standard 500/2500 sets with the "G" (round) handsets. So, you put the handset in the cups, dialed your number, and started communicating. Early modems worked the same way (hey 300 baud was FAST).