I have USB->serial adapter:

uplcom0 at uhub2 port 1
uplcom0: Prolific Technology PL2303 Serial adapter (ATEN/IOGEAR UC232A), rev 1.10/2.02, addr 2
ucom0 at uplcom0

It works fine when I connect it to 0-modem cable and that cable to serial port of server.
Also I have phone with serial interface.  This phone works just perfectly when
I connect it to serial port of server.
But when I try to connect phone to USB->serial adapter it does not work.
I tried to directly tip(1) to /dev/ttyU0 -- it gets nothing, or sometimes ^@.
Speed matches.

The most strange thing is that 1 time out of 100+ attempts this connection
USB->serial->phone worked.  I successfully established ppp session.
But all other times it does not work.

Anyone have idea why it can not work?