There are basically 3 ways roaming voice call are handled :
1) Free : only Simba & eight
2) Pay per use - you pay through the roof to answer a call :
All MNO except Simba,
Singtel based MVNO that are not directly owned by Singtel,
Starhub based MVNO that are not directly owned by Starhub
Circle.Life
If you do not intend to answer & pay for incoming calls while roaming using this group of providers, recommend enabling
redirect ALL calls to another number that can either answer for free or can ring without concern.
3) Roaming calls are blocked, there is no option to receive a roaming call :
MVNO owned direct by Traffic light telco (except eight) - Gomo/Heya, Giga,Maxx,
Changi Mobile & Geenet.
Gomo
FAQ did mentioned that (pay per use ?) roaming on 3G is still available, it is unclear if that applies to Heya/Giga/Maxx.