Step 1:- Go to http://www.mobivox.com and register there for free account.

Step 2:- During registration, remember to insert your friends (Victims) mobile number in "Phone number" field as shown
below.

Step 3:- Complete registration and confirm your email id and then login to your account.

Step 4:- Click on "Direct WebCall" After successful Login into your Mobivox account. as show below.

Step 5:- You will arrive at page shown below. In "Enter a number" box, select your country and also any mobile number(you can enter yours). Now, simply hit on "Call Now" button to call your friend with his own number.

Step 6:- That's it. Your friend will be shocked to see his own number calling him.

Remember

[1] .You get only 10 min to call free after that you need to pay money , but you can makeanother account with another friends number and another email id and start pranking again
[2] .But don't miss use this hack by calling someone's GIRL Friend(s) OR BOY Friend(s). Because this hack is untraceable. If You call Customer Care and tell about this then they will tell this thing cannot happen.