Just paid my Maxis bills via Maxis-Pay-by-Phone today and was informed that effective March 2015, Maxis bills can now be paid using AMEX via Maybank2u (previously cannot)

No need to get their callback during weekends anymore

P/s: If you previously (before March 2015) added Maxis bill as favourites, it won't appear under AMEX when you do Favourites Bill Payment. You have to delete the bill from favourite and re-add it, then it'll now appear under AMEX when you perform Favourites Bill Payment.