It shouldn't make any difference whether it's Lucent or Rockwell. As far
as 56k interoperability is concerned, presently, there are only a few
things to remember:
K56Flex works with K56Flex (Rockwell and/or Lucent)
X2 works with X2 (USRobotics)
K56Flex does NOT work with X2 (and vice/versa) beyond V.34 speeds.
One of the most important things to pester people about is keeping their
modem's firmware up-to-date. If the modem is K56Flex-based and has a
firmware version that is BELOW 1.0, he/she WILL experience problems
connecting and maintaining a reliable connection to any K56Flex-based ISP.
USR modem users aren't exempt either. If their firmware isn't up-to-date,
they'll experience what USR calls "pausing".
Having the latest firmware in the user's modem AND your NAS will make all
the difference in the world...
There's some more info (and links) to 56k-related stuff at: