Supplementary information to the ISDN address assigned
to this signaling channel. Usually, this is the
subaddress as defined in Q.931.
If there is no such number defined or required, this is
a zero length string.
The subaddress is used for incoming calls as well as
for outgoing calls.
The subaddress is an user supplied string of up to 20
IA5 characters and is transmitted transparently through
the network.