You might be in a destination where no 3G network is available i.e. Australia, Japan, Puerto Rico, Taiwan or USA. Check these settings to make a call.

1. Log in to My Singtel app and tap on Roaming. 

2. Tap on the gear icon and enable Data Roaming (Network).  

3. If you are using a travel eSIM from other operators, go to your device settings and Turn on this line and Mobile data.

If you also have a Singtel SIM, make sure that you've turned off Data roaming in your device settings. 

Note that pay-per-use charges will apply for any calls made.