Important things to know first

To quit modemu2k from a command prompt, type ATQ. Ctrl-C does not abort modemu2k.
When running modemu2k under a comm program (e.g. minicom), use that program's normal quit key — modemu2k exits along with it.
modemu2k defaults to a 7-bit cooked connection. For 8-bit-clean transfers (ZModem, Kermit, binary protocols, BBSs that need raw bytes), enable binary mode with ATB0=1B1=1&W. See the B description in man modemu2k.
Dialing takes a quoted address. To specify a non-default port, use a space between host and port (not a colon):

atd"bbs.example.org 2030

Standalone (stdin/stdout)

To talk to modemu2k directly without a comm program, invoke it with any flag — for example -e to run a startup AT string:

modemu2k -e "AT"

modemu2k reads AT commands from stdin and writes responses to stdout. Dial out with:

atd"localhost 8023

You'll see the remote side's prompt. On disconnect you get NO CARRIER. Quit with:

at%q

With a comm program (the usual mode)

Tell modemu2k to fork a comm program and hand it the PTY slave path via s:

modemu2k -c "minicom -l -tansi -con -p %s"

For 8-bit-clean transfers, run the startup AT string at the same time:

modemu2k -e "AT%B0=1%B1=1&W" -c "minicom -l -tansi -con -p %s"

A convenience wrapper, m2k-minicom, runs the above invocation for you. It's in scripts/m2k-minicom; install it by configuring with -Dhelper-scripts=true before ninja install. A matching m2k-picocom ships under the same option.

Any arguments you pass to the wrappers are forwarded to modemu2k. For example, to enable verbose narration while running through minicom, redirect stderr to a log file (modemu2k refuses to run otherwise, because verbose output would corrupt the comm program's display):

m2k-minicom -v 2>/tmp/m2k.log # then tail -f /tmp/m2k.log

From inside minicom (or picocom, etc.), dial in the same AT style:

atd"trmb.ca 2030

To quit modemu2k, just quit the comm program — modemu2k tears down with it.

Listen mode (incoming connections)

Run modemu2k as a TCP listener that uses the accepted socket as its TTY. The connecting client drives modemu2k with AT commands. Intended mainly for plugging into emulators like dosemu2.

modemu2k -l 19876

The process accepts one connection on port 19876, then runs the modem state machine over it until the peer disconnects.

Hanging up and escaping

Escape to command mode while online: type +++ (with the modem-style timing pause). modemu2k will return to command mode and display OK.
Return to online mode: ATO.
Hang up the current connection: escape to command mode, then ATH.

Where to look next

man modemu2k — full option reference and AT command list.
README — project overview, building, downloads.
API documentation — generated from modemu2k.h doc-comments, published at https://theimpossibleastronaut.github.io/modemu2k/.