Are you using a 3G / non-VoLTE phone?
Please check that you are not using a 3G or non-VoLTE phone. As 3G network is no longer supported in Singapore, you will need a 4G/5G VoLTE phone to connect to the network. Depending on the phone model, you may also need to enable VoLTE in the phone Settings.
More info on 3G shutdown here.
Is your phone's Do Not Disturb mode TURNED ON?
Please check if your phone's do not disturb mode is turned on. If so, please help to TURN it OFF.
For iphone : https://support.apple.com/en-us/HT204321
Samsung : https://www.samsung.com/us/support/answer/ANS00077281/
Is the caller's number blocked?
Please help to check if the caller's number is blocked by your mobile phone. If so, please remove it.
For iPhone : https://support.apple.com/en-us/HT201229
For Samsung : https://www.samsung.com/au/support/mobile-devices/block-calls/