If one t38modem instance is used for many fax numbers (in case if all numbers are from same sipgw/port, not each one on different port), adding new modem/new fax number requires stopping and running again new instance of t38modem with different parameters. Not exactly best thing to do in production environment, even if changes can be accumulated and performed at less often intervals as temporary workaround. With many many numbers it takes time to restart it and reinitialize all the modems, during which some send/recieve may be interrupted.
Would be nice to see ability to check/validate parameters (in case if there is syntax error) and add new modems/allocate new pty-s or apply other configuration directives without restarting t38modem instance. For validation probably one can also script up some shellscript, if putting that into t38modem executable is not too feasible.
Something like for other unix daemons (eg. testparm for samba and kill -HUP PID to reread configuration).
Also maybe it's worth to implement not just CLI parameters, but also configuration file that t38modem can use? (+what's told above about configuration validation testrun and rereading config to add new modems without restart/reinitialising of old ones).