US T-Mobile will end it’s 3G network soon. What that means is that 3G phone users will have to upgrade their phones to 4G to use the services including VoLTE voice calls.
In Singapore, the 3G networks spectrum rights will expire by end of 2021. Similar to T-mobile, existing 3G smartphones wont work and 4G phones without VoLTE won’t be able to make any voice calls.
Although IMDA specified that the 3G network spectrum rights will expire by end of 2021, it may not shutdown or even extend it for another few more years while deployment of 5G networks carries on.