Christoph writes:
> > TD was flickering (for a while)
> >until the modem deasserted DTR.
> Ahem.  The modem does not deassert DTR.  DTR comes from the DTE.

You're right, I had the problem exactly backward.  The modem left CTS
deasserted, which (presumably) meant that the PPP test packets weren't
getting through so pppd killed the connection eventually.  Never mind.
(What's odd, though, is that TD *was* flickering, indicating that NetBSD
*was* sending with CTS deasserted.  crtscts *is* enabled on the tty line.)