MACRO:
exten => s,1,Background('transfer')
exten => s,n,GotoIf($["${ARG3}" != ""]?s,gotvm)
exten => s,n,Set(VMBOX=${ARG2})
exten => s,n(gotvm),Set(VMBOX=${ARG3})
exten => s,n,Dial(${ARG1}/${ARG2},17,m)

RESULTS:
-- Playing 'transfer' (language 'en')
-- Executing GotoIf("Zap/22-1", "0?s|gotvm") in new stack
-- Executing Set("Zap/22-1", "VMBOX=2010") in new stack
-- Executing Set("Zap/22-1", "VMBOX=") in new stack
-- Executing Dial("Zap/22-1", "SIP/2010|17|m") in new stack

ANY IDEA WHY?