I have a DSC alarm system hooked up to this modem and when I send the code for the alarm company to call the panel to do the programming the modem's LEDs all flash except line 1 (I think).
Oddly it works fine for incoming/outgoing calls.
I am assuming that the alarm company sends some "fax type" sounds and conclude this could be confusing the modem. Another thought is if voice mail is picking up before the panel can answer, is the flashing of the LEDs to indicate a voice message is being recorded??