If you are using any of the Singtel, M1 or Starhub Multi-SIM cards, you will have 2 or more SIM card. However only 1 will be the Primary SIM card. The rest will be Secondary SIM cards.
Primary SIM cards are able to call out and
receive calls
Secondary SIM cards are only able to call out
In order to switch any card to a Primary SIM card, please
key in the following in dialer:
SingTel: Dial
*141# and press call.
StarHub: Dial *118# and press call.
M1: Dial *136# and press call.
StarHub: Dial *118# and press call.
M1: Dial *136# and press call.