Telebit T3000 Reference Manual page 160

Call in Progress (continued)
If the answering site's modem is configured to ignore the
DTR signal (&DO), it maintains the connection even if its
DTE drops DTR because of inactivity or some other
condition. If this happens, the calling site must terminate
the connection and, if desired, reinitiate the call.
Was the connection dropped prematurely?
If the modems connect (CD indicator is lit), the carrier
signal may have been dropped because of a degradation in
the line quality of the connection. Try placing the call
again. The probability of getting the same connection is
unlikely.
If the modem's &D command is set to 2 or 3, and its
associated DTE drops DTR because of inactivity or some
other condition, the modem disconnects the call.
Troubleshooting 7-17