1: Almost all carriers support volte, worldwide. But many only support for their own customers, not for roamers.
2: This is not a problem for Singtel, StarHub, etc. users, because when they are roaming and making calls, using volte or traditional 3G to call, all have charges, no difference.
3: volte needs phone hardware support, and OS support, and carrier support. Samsung phones usually only work in some regions, not all, due to their CSC region lock. For example, if you bring your SG Samsung phone to Europe, even if your phone chip and hardware are the same as European sets, your phone software's CSC lock won't let you do volte calls there.
iphone's do not have this problem. All iphones, SG, CN or US, all share same volte capability and same carrier bundle.