Instructables
This is a glitch me and my brother found out. It is pretty dang loud to, so be prepared. It also kinda sounds like a Ring Wraith from Lord of the Rings. And all it requires is two cell phones!(We did it with a RAZR and a Samsung Sync, it might now work with other phones.)
 
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Step 2: Call

Picture of Call
Now call the other phone, and pick up.
This shouldn't be to hard, unless your like my brother. =)

Step 3: Call again

Picture of Call again
Now call that same person again with the same phone, do not hang up the first call, and pick it up also. So now you should have 2 calls on the line.