https://howto.org/block-outgoing-calls/ lists many methods.

— inside the telco.. One site says “Most phone services block outgoing international calls for both landline and mobile services upon request.”

Starhub said on 6 Sep 2021 that they could block outgoing IDD internally. This is the most reliable solution in my experience.

— Android feature callBarring.
I was able to turn on callBarring for “InternationalOutgoingCalls”.

The same screen also lets me update the callBarring passcode, currently 1890

Tested: barring All incoming including local
Tested: barring All outgoing including local .. phone says “Your service does not allow outgoing…”

— Q: In Singapore, do you EVER want to enable outgoing IDD ?
For 20 years I have never needed it. IDD is too expensive. I have multiple alternatives:

• free IDD in office and home
• calling cards
• whatsapp, wechat
• There is only one Starhub add-on i.e. the 018 dialing method. I never enabled it.