A Directory Number. Directory Numbers are used
to identify incoming calls on the signaling
channel given in isdnDirectorySigIndex.
The format of this information largely depends on the type
of switch or PBX the device is connected to. Therefore,
the detailed format of this information is not
specified and is implementation dependent.
If possible, the agent should implement this information
using the E.164 number format. In this case, the number
must start with '+'. Otherwise, IA5 number digits must
be used.