If you are the early adopters of Zero1, you wouldn’t be able to use voice and SMS roaming with your existing SIM card. Zero1 will be replacing the old SIM cards for those who activated their lines before 15 Aug 2018 in November 2018.
If your line is activated after 15 Aug 2018, especially those who signed up for the S$9.90 plan, you will be able to voice roam in other countries. Below are the pricing information for Malaysia, Taiwan and China.
If you are entering Malaysia, the likely roaming partner is Maxis.