SIP Rule to Insert Data into method CANCEL

rule invite_and_cancel
when RULE_INIT {
    array set ::callidtable{ }
}
when CLIENT_DATA {
 Get the UDP Header, Payload, and Call-ID info
set header [UDP::payload 48]
set fullpacket [UDP::payload [UDP::payload length]]
set callid [findstr $fullpacket "Call-ID:" 9 "
" ]
 Check if this is a SIP Method INVITE and the FROM field is "anonymous"
if { $header contains "INVITE" } {
   if { $fullpacket contains "From: " } {
 Find the P-Asserted-Identity field and insert it into the array
 This next command MUST have the carriage return in it to pick up the right amount of data.
    set ::callidtable($callid) [findstr $fullpacket "P-Asserted-Identity:" 0 "
" ]
    return
    }
   return
}
 Check if we've previously stored this Call-ID and this is a CANCEL
if { [info exists ::callidtable($callid)] } {
   if { $header contains "CANCEL"} {
 Make the P-Asserted-Identity header insert at the right place
       set newlength [UDP::payload length]
       incr newlength -2
       UDP::payload replace $newlength 0 $::callidtable($callid)
 Insert a double "0d0a" to end the payload correctly
       set newlength [UDP::payload length]
       incr newlength -2
       UDP::payload replace $newlength 0 "\n"
 Clear the array for this callerid
       unset ::callidtable($callid)
       return
      }
   else {
  If we're here, we're seeing a different message than INVITE or CANCEL
       log "Message other than INVITE / CANCEL for CALL-ID $callid"  
         }
   }
}