Very often, i found myself(Singtel User) wasting my call time on M1 users, especially those that comes with voicemail. Each time, your call enters the voicemail part, your 1minute gets automatically deducted off, or if you are Starhub or M1 user, a few seconds.
It's really frustrating trying to call M1 users who doesn't deactivate their voicemail services and it just kept eating up your FREE MINUTES, worst your 1minute of call time multiply by the amount of time the call ends up to the voicemail service.
Hence, i tried this on my brother's M1 line and it works.
To deactivate all calls, press "##21#" CALL. It will deactivate the voicemail service.