Siemens HiPath 1100 Series Programming Manual page 74

DISA
Step by Step
74
Only one external call can be made or one
feature can be used at a time.
A feature that is enabled during a call is disabled
as soon as one of the parties hangs up.
TAPI only monitors physical ports. To operate
correctly, a DISA feature must use special ports,
and those cannot be monitored. If a physical port
is used when the DISA feature is active, the TAPI
will be able to monitor it.