(T) customers can ditch their AT&T phones and use any wireless phone, device and software application from any maker — think smartphones, e-mail and music downloading. And they don't have to sign a contract. "You can use any handset on our network you want," says Ralph de la Vega, CEO of AT&T's wireless business. "We don't prohibit it, or even police it." AT&T's push to give consumers maximum cont