How to turn off permanently voicemail for Zain prepaid?

Hey guys

How can I turn off the voicemail feature on my Nokia C2 mobile? It's not a smartphone. It's on Zain prepaid network with Khateer package.

I went the Zain desk at the Mall, the guy said he turned it off by pressing some buttons on the phone. I didn't see what he did, but it took 3 seconds. I didn't get any voicemail for 2 months which was good but now people are leaving me voicemail again.

How do i permanently turn off voicemail?

Thanks

Dial ##002# by your zain prepaid or postpaid sim.