${CDR(accountcode)} * - Account code (if specified)
  ${BLINDTRANSFER} - The name of the channel on the other side of a blind transfer
  ${BRIDGEPEER} - Bridged peer
  ${BRIDGEPVTCALLID} - Bridged peer PVT call ID (SIP Call ID if a SIP call)
 ${CALLERID(ani)} * - Caller ANI (PRI channels)
  ${CALLERID(ani2)} * - ANI2 (Info digits) also called Originating line information or   OLI
  ${CALLERID(all)} * - Caller ID
  ${CALLERID(dnid)} * - Dialed Number Identifier
  ${CALLERID(name)} * - Caller ID Name only
  ${CALLERID(num)} * - Caller ID Number only
  ${CALLERID(rdnis)} * - Redirected Dial Number ID Service
  ${CALLINGANI2} * - Caller ANI2 (PRI channels)
  ${CALLINGPRES} * - Caller ID presentation for incoming calls (PRI channels)
  ${CALLINGTNS} * - Transit Network Selector (PRI channels)
  ${CALLINGTON} * - Caller Type of Number (PRI channels)
  ${CHANNEL} * - Current channel name
  ${CONTEXT} * - Current context
  ${DATETIME} * - Current date time in the format: DDMMYYYY-HH:MM:SS (Deprecated;                   
  ${DB_RESULT} - Result value of DB_EXISTS() dial plan function
  ${EPOCH} * - Current unix style epoch
  ${EXTEN} * - Current extension
  ${ENV(VAR)} - Environmental variable VAR
  ${GOTO_ON_BLINDXFR} - Transfer to the specified context/extension/priority after a blind                
  ${HANGUPCAUSE} * - Asterisk cause of hangup (inbound/outbound)
  ${HINT} * - Channel hints for this extension
  ${HINTNAME} * - Suggested Caller*ID name for this extension
  ${INVALID_EXTEN} - The invalid called extension (used in the "i" extension)
  ${LANGUAGE} * - Current language (Deprecated; use ${LANGUAGE()})
  ${LEN(VAR)} - String length of VAR (integer)
  ${PRIORITY} * - Current priority in the dialplan
  ${PRIREDIRECTREASON} - Reason for redirect on PRI, if a call was directed
  ${TIMESTAMP} * - Current date time in the format: YYYYMMDD-HHMMSS 
  ${TRANSFER_CONTEXT} - Context for transferred calls
  ${FORWARD_CONTEXT} - Context for forwarded calls
  ${DYNAMIC_PEERNAME} - The name of the channel on the other side when a dynamic feature   is used.
  ${DYNAMIC_FEATURENAME} The name of the last triggered dynamic feature.    
  ${UNIQUEID} * - Current call unique identifier
  ${SYSTEMNAME} * - value of the systemname option of asterisk.conf
  ${ENTITYID} * - Global Entity ID set automatically, or from asterisk.conf