Voice Roaming charges can be a lot for Singtel and ST MVNOs and there are multiple discussions on how to avoid the charges in this forum.
Most of the users will not get charges if they ignore the incoming calls but there are reported exceptions. Same for rejecting the call, in many cases there will not be charges but there are exceptions as well.
Singtel roaming terms and conditions mentions that rejecting the call may incur charges, but in reality many report no charges as well. So it may depend on the network.
Interestingly it mentions that US/Canada operators will charge after 30 seconds of ringing. So for those two countries, better to reject the call.
The following Singtel Terms and Conditions may be worth reading.
https://cdn2.singteldigital.com/con...s-services/mobile/roaming/pdf/RoamingTnCs.pdf