[100]
;get current number if exists for extension
exten => 456,1,Set(CURRENTNUMBER=${DB(100/NUMBER)})
exten => 456,n,GotoIf(CURRENTNUMBER = ${"456"}]?HANGUP:MAKECALL) 
exten => 456,n(MAKECALL),Set(DB(100/NUMBER)=456)
exten => 456,n,DIAL(SIP/456,20,gWW)
exten => 456,n(HANGUP),HangUP();