How to dial-in/out on a single tty line?

[Jeff Lo]

We just moved all of our mail/news stuff from a MicroVAX II to a
SPARCserver 330 running SunOS 4.1.  We are using Telebit Trailblazer+ and
USRobotics Courier 2400 modems on the 4 standard on-board serial lines.
We followed the modem setup instructions in the System & Network
Administartion manual and are able to dial out just fine if the gettys are
off, and dial in if the gettys are on.  The problem is we would like to be
able to dial in and out without switching the gettys off and on.  The
manual appears to indicate that this is possible, but we haven't been able
to get it to work.  This is possible, isn't it?  Any suggestions?