You've got the wrong number — there's no Mr. Zhang here.
Tips
usage
打 covers a wide range of actions including dialing a phone (打电话) and typing on a keyboard (打字), so 打错了 can mean either depending on context. On the phone it's the standard 'wrong number' apology.
mistakes
Don't translate 'wrong number' as 错号 — that doesn't work. The verb-complement structure 打错(了) is the natural form.
