${DNID} Dialed Number Identifier
${RDNIS} Redirected Dial Number ID Service
${HANGUPCAUSE} Hangup cause on last PRI hangup
-${SIPDOMAIN} SIP domain (if appropriate)
+${SIPDOMAIN} SIP destination domain of an inbound call (if appropriate)
There are two reference modes - reference by value and reference by name.
To refer to a variable with its name (as an argument to a function that