My guess is that you're using eFax. Since I'm already sure about your answer, I want to hit you with a couple of related questions:
1. Are you seeing anything else reported along with that "wrong response to command", such as +FTH=3 or +FRH=3?
2. Are you using an LTmodem (formerly Lucent Technologies...now Agere)? That chipset is not supported under Linux.
3. Have you tried using HylaFAX's faxgetty? Provided that you're using a supported fax/modem, HylaFAX should do the trick.
There's a nice collection of Linmodem links
here if you're not sleeping well. It's good stuff.