Mobile calls random numbers?

I just got an iphone 4s on an optus plan.
I have had a recurring problem when making voice calls. I will select a contact from my contact list and press "call". The phone then says… "calling" but I'm connected to a random number, not my contact.
this has happened several times. What is going on?

It seems the phone is a defect. Take it back to AT&T and get another one.

Maybe its a refurbished? I would take it back and have it checked out…

Hi, take it to At&T lab to tackle this problem cause this is some software problem which is not functioning well.
First call to customer care to get good suggestion.

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