Here is a really great page with all sorts of AT-related commands:
Category: POTS lines (modems, FAX, etc.)
USR V.Everything Modem Commands
article #264, updated 2258 days ago
Replacement for HyperTerminal
article #150, updated 2558 days ago
A good replacement is here, called Tera Term:
It handles some modems which PuTTY won’t, for instance, the Zoom Model 3095 (USB Mini External).