Either one is ok. We know exactly when the person called the wrong number - it was when you heard the phone ring, right before you picked up. The present perfect tense ("you've called") is fine because you know this action was in the recent past, and the present perfect is commonly used for recent past actions. The simple past tense ("you called") is also fine because again, we know this action happened in the past (the recent past, but still the past). There is only one past event here, so you don't have to worry about using different tenses to make the order of past events clear, which is often a reason to use different tenses. For instance: "You called me yesterday, at which time I told you this was the wrong number, but you've called me ten more times since then!" - in this case, the present perfect indicates an ongoing action that might still be happening now.
EDIT: I will also say that I agree with David Siegel that "Sorry, wrong number" is a more common response when someone calls you accidentally.
EDIT 2: Mazura also has a good point - you are probably more likely to apologize if you are the one who has dialed incorrectly.