How can I stop someone hacking into my contacts and text messages?

How do you know he can read your text messages, see your contacts, and see your call history?

Did he have access to a PC to which you had synced the iPhone? If so, he could have pulled the info he's using from there.

The most robust solution would be to get your iPhone phone number changed - that wouldn't undo the fact that he has your contact list, but could stop him from harassing you.

It's a pain to let people know you changed your number, but cheaper than paying a lawyer to get an injunction (which you probably could also do)

There are programs out there that allow you to spoof your phone number when calling or texting someone, so that they think it was someone else. As nick101 stated, if he had access to your computer in the past, then he could have pulled your contact list and is now using it. If you don't have anti-virus and a firewall setup on your computer, I highly suggest you do so. It could be possible he has access to your computer, but I highly doubt he hacked into your iPhone, even if he is computer savvy. It could be possible he setup a VPN or some remote desktop application on your computer that is running in the background, which would make it possible for him to easily connect to your computer.

If you really want this to stop the cheap way, change your phone number and hire someone to install a firewall and anti-virus, and also have them look at your computer for any remote desktop applications or any other openings that would allow a person to access your computer. You could also speak with a lawyer about this or talk to the police and see what they can do about this.

Well this is very bad. I don't see how he could be getting into your iPhone. Most likely something from your computer. The first thing you should do is make sure any networks you are on are secure. Make sure your wifi has a password. Make sure the firewall on your PC is turned on or get a third party one and anti-virus/malware to check for anything that may be on there.

Second, document the calls and texts he sends to you and call the police.

You also need to call AT&T and provide them with this information.