For a server answering many calls (a BBS, PPP dial-in or fax), dmodem-ari beside Asterisk (frontend/ari/), the Asterisk module in frontend/asterisk/ built for your own Asterisk, or the FreeSWITCH module in frontend/freeswitch/ is the better fit than dmodemd -L: it answers with the open modem at V.91 (another digital modem calling, up to 62667 bps both ways), V.90 (as the digital modem, up to 56000 bps downstream), V.34, V.32bis, V.22bis, V.22 or V.21.
Tested over SIP against a local Asterisk and a local FreeSWITCH only, not yet through real PSTN gateways.
Of V.92, the digital modem has quick connect (short phase 1) and short phase 2 for V.92 analogue modems calling, PCM upstream (36000 bps upstream in the tests, against 31200 for V.34; DSP_V92_PCMUP=0 leaves it out) fast parameter exchange and modem-on-hold (it holds the call for an analogue modem taking a waiting call, and picks it up again; AT+PMH, AT+PMHT; and AT+PMHR asks the analogue modem to go on hold, though the modems we test with retrain instead). V.34, V.90 and V.92 calls renegotiate their rates when the line gets a little worse or better (both ways: ours, or the analogue modem's), and retrain after a line hit or a large change.
V.90 needs an analogue modem at the other end: dmodemd is always on the digital side of the network. Two digital modems (D-Modem calling the Asterisk module, or another D-Modem) connect at V.91 instead: up to 62667 bps both ways on a clean G.711 call (about 58700 bps of data through V.42), less where the network robs bits or adds a pad; AT+MS=91 asks for V.91 (or V.34), the default AT+MS=92 offers it with V.90.
The serial interface could be replaced with stdio or a socket, and common AT configuration options could be exposed as command line options