Users of Apple Inc.'s popular iPhone may now be able to save money by making Internet-based phone calls over AT&T Inc.'s cellular network.   Apple this week allowed new versions of several Voice over Internet Protocol services to begin working on the iPhone, according to those services. Previously, iPhone users needed a wireless Internet connection to make such calls, but the change will allow calls from anywhere that receives a strong enough 3G cellular signal.     By using VoIP applications to sidestep the phone's normal calling software, iPhone