It was drop DTR. Most current documentation tells you that dropping DTR means the modem should hang up. That was an option, maybe even the default. But you could AT&D1 to make dropping DTR return you from data mode to command mode. It's so important there's even a DIP switch on the USR Courier to enable this mode.

https://support.usr.com/support/3453b/3453b-crg/chap%208-con...

I almost can't believe the USR site is still up. This is something I remember doing to run a BBS over 30 years ago. I still have my Courier with the brass not for resale plate.