I think you're using a keyword which is for Prepaid subscribers..
for postpaid, keyword should be MYSUPERDUO MM ON send to 8080.
if you are subscribed to myLifeStyle Plan, it is best that you call 211 or (02) 730-1000 to activate the Duo Pack 299 on your account. SuperDOU is not applicable for mylifestyle plan since base plan is already unlimited calls and texts to Globe/TM numbers.
BTW keyword for DOU postpaid is --> MYDUO MM ON worth 299 for postpaid.