StarHub's promotions page has an 11.11 festive banner promising $0 phones and 50% off plans. The page gives no dates or full terms, so confirm details before you sign up.
Singtel's 11.11 Online Flash Sale lists 25% off monthly mobile plans, 25% off SIM Only plans from $30/mth with port-in, and 50% off its new 10Gbps Lite broadband plan, all until 15 November. T&Cs apply.