Singtel, M1, Starhub Multi-SIM - Setting to Primary SIM

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.