If your application is HylaFAX, there is a HylaFAX modem configuration
file included with IAXmodem (config.ttyIAX).  The config file is for
HylaFAX version 4.2.2 and later.  Instead of running faxaddmodem,
simply edit this config file to your liking and then copy it to
/var/spool/hylafax/etc/.  (Run faxgetty and the rest of HylaFAX as
usual.)