Does your screen go black when the phone is near your face? (Use a mirror, or have someone look.) If not, you're "cheek dialing". If it does, bring the phone into the carrier's store, ask them to place a call and explain to you why it'a happening.
(Unless you're talking about hearing a fast series of tones about a second after you dial your call. That's an old telephone company central office, and sloppy maintenance on their part - you shouldn't hear the order tones.)