VA4224 won't talk to 3B1

[System Mangler]

Our Vadic VA4224 rack-mount dialout, which works fine with over 25
UUCP sites, refuses to originate a connection to an AT&T PC7300.

I dialed the modem by hand from a terminal while listening on a
telephone handset; the remote PC7300 would answer, and whistle,
then the familiar 1200-bps squalling sound came on and the modem
reported "ONLINE".  A second later, the phone hang up.	It was
consistent, every time, and no login prompt ever appeared.

My Vadic Maxwell 2400V talks to this PC7300 just fine.	The
VA4224 uses the same chipset (Rockwell), why can't it talk too?
A Rev-level problem?  (VA4224 has Rev. F, 2400V has Rev. G).

What am I missing?  Is it one of the myriad switches/straps?
There are about 2^30 combinations...

Don Speck   speck at vlsi.caltech.edu  {seismo,rutgers,ames}!cit-vax!speck