Sip invite phone number

i want to send phone number in a sip invite …like this
does asterisk can recognize that this is a request for a phone and not for a sip uri
i try : exten => _X.,1,Set(result=${ENUMLOOKUP(+${EXTEN})}) but
it seem that EXTEN is the src sip request …and it looking’s for extension definition for 0494212181 instead of check the dns record

any idea