If you’re like us and juggle between multiple SIM devices, here’s a quick reference guide to designate a specific SIM card as your primary SIM to calls and SMS to come through, you’d need to dial in the following into your phone depending on your telco in Singapore –
M1: Input *136# and press dial
SingTel: Input *141# and dial
StarHub: Input *118# and dial
Hope this quick guide was of help!