soft, single ringing phone sound

Hi, we have a strange problem, hoping for some help.  New Inspriron desktop has a soft phone ringing sound.  It's random, about once a hour, and when it rings twice (which is rare) the mouse freezes.  Tried all kinds of things with no luck - including turning off all Windows sounds.  If we turn off our external speakers it stops which someone once suggested (back in 2003) would indicate a modem is involved - not sure of that logic.Thanks, Ed