FaceTime just keeps getting better. The recent addition of audio calls in iOS 7 is great news, right? Well, sort of.

There are plenty of apps in the App Store that let you make calls over your data connection rather than through the carrier’s phone network.

FaceTime audio calls are great — something that Google+ Hangouts have had for a long time. (Hangouts actually lets you add a voice call to a group video Hangout.) They enable free international calls, for starters. The protocols underlying FaceTime enable high-quality audio calls.

More importantly, they give users one more reason to get into the FaceTime habit.

Unfortunately, FaceTime has a fatal flaw. It’s still — inexplicably — an exclusive phone system for Apple customers to call each other. What kind of phone system is that?

FaceTime has been around for three years now. When Steve Jobs announced it in the summer of 2010, he promised that Apple planned to make FaceTime an open standard so that other companies could make FaceTime-compatible apps and services. And that made perfect sense.

FaceTime is actually built on a variety of open standards. Unfortunately, Apple prevents other companies from implementing FaceTime support.

From the user’s perspective, FaceTime should work like iMessage. If Apple did it right, you’d be able to simply use it to make all your calls — video and audio. If the person you’re calling has FaceTime, great! If they don’t, that should be when the phone fails over to either the regular carrier’s phone system or another VoIP alternative. Or, it would connect to the other user’s FaceTime-compatible app.

iMessage is great because the user doesn’t even have to know or remember what kind of phone the other person has. It just works. If the connection is iMessage-to-iMessage, then Apple confers special benefits, not the least of which is monster privacy and security. If the connection is iMessage-to-SMS, that’s fine, too. It’s a regular text session. Either way, you can use iMessage every time without thinking about it.

This is the very feature that makes iMessage superior to Google’s Hangouts app. While Hangouts can be used on Android, iOS or on any PC running Chrome or Google+, it doesn’t interoperate with SMS. So if the other user isn’t using Hangouts, you can’t connect with them.

The same flaw exists with FaceTime. You have to think about what brand of phone the other person has before you make a call. Gimme a break.

If FaceTime doesn’t work like iMessage, at least it should work like Skype, where you can go user-to-user or user-to-nonuser. Apple could even charge for that service. I’d pay.

I think it’s reasonable for a giant technology company like Apple to keep features exclusive to their platform as a way to incentivize new customers and hang on to existing ones. For example, Apple’s Siri virtual assistant service is available exclusively to users of Apple’s products. You can’t download the official Android version of Siri, because it doesn’t exist.

But it doesn’t matter, because Siri users can still interact with non-users via Siri. For example, you can tell Siri to send a message or make a call or schedule a meeting with a non-user, and it works great.

FaceTime’s exclusivity is different. You can’t use FaceTime to call a non-FaceTime user. As such, it’s not a super compelling benefit to embrace Apple products. It’s nice. But nobody is going to place FaceTime on their short list of reasons why they choose Apple over alternatives.

I would also assume that FaceTime could be one of the “killer apps” for either Apple’s rumored upcoming TV or a future version of the Apple TV box or both. Video calls where you’re sitting comfortably in your living room and seeing your family and friends on the TV makes a lot of sense.

But if those calls can work only with other Apple users, the killer app itself is dead.

It also needs to be said that Apple’s move — as limited as it is — it another bit of overwhelming evidence that the wireless carrier model is obsolete. Instead of capping and throttling and punishing high-speed mobile broadband usage while simultaneously trying to threaten and negotiate and force and trick the usage of SMS and phone functionality, carriers need to do the opposite. They need to kill the phone functionality and increase the speed and bandwidth of wireless data connectivity by an order of magnitude.

As Apple is proving, and others have proved, a phone call is just another app.

What we really need is one app — not several depending on which phone each users has access to.

It’s time for Apple to live up to Steve Jobs’ promise and make FaceTime an open standard.