Google on Tuesday plans to enable a subset of Google Voice features, notably voicemail, when using an existing mobile number, a move that partially thwarts Apple's effort to keep Google Voice off the iPhone.

Google Voice provides a variety of calling services in conjunction with a new Google Voice number. The service is designed to receive calls to the user's Google Voice number and then ring all of the existing phone numbers supplied by the user to connect the incoming call.

It's an effective way to never miss a call. But Google's approach has proven to be a problem for users who don't want a new Google Voice number to worry about.

It has also proven to be a problem among those who want to make outgoing calls using their Google Voice number. That's because a mobile phone will identify itself with its carrier-supplied number rather than a Google Voice number. This isn't ideal because those called can become confused about which number to call to reach a Google Voice user.

Over the summer, Google released Google Voice apps for Android and Blackberry mobile phones to allow outbound calls using one's Google Voice number. These apps provide a consistent phone identity for inbound and outbound calls.

But Google's ambition to improve Google Voice on the iPhone has been blocked by Apple, which has refused to approve a Google Voice app for the iPhone.

"The application has not been approved because, as submitted for review, it appears to alter the iPhone's distinctive user experience by replacing the iPhone's core mobile telephone functionality and Apple user interface with its own user interface for telephone calls, text messaging and voicemail," Apple explained in a statement posted on its Web site.

Apple's decision, a matter the FCC has been looking into, has left iPhone users unable to fully utilize Google Voice.

But with Tuesday's announcement that Google Voice users can use Google Voice voicemail instead of carrier-supplied voicemail, Google has effectively found a backdoor into Apple's iPhone fortress, through poorly defended mobile carrier network infrastructure.

Google Voice product manager Vincent Paquet says that Tuesday's announcement isn't directed at Apple. "It certainly wasn't the intent," he said. "This feature was available before Google Voice came out so it's not like we created this to bypass anything. What we've come to realize is that a subset of our users will not want to change numbers and we understand that." Indeed, it's fair to say that Google is trying to empower Google Voice users. Consider the way a Google-produced video frames an advantage of having Google Voice transcribe one's voicemail messages: "If you ever change your phone company, you can take your messages with you," the video explains. "They are yours after all."

Another way that Google could put it would be to say that voicemail messages are not the property of mobile carriers, but that might sound like an effort to disempower mobile carriers.

The "feature" Paquet cited is called "conditional call forwarding," which has been available on phone networks for years. As its name suggests, it allows calls to be forwarded to another number under certain conditions.

What Google has done is given users the ability to set that condition for incoming calls. Now, if a Google Voice user chooses, he or she can send calls to Google Voice's voicemail system rather than to, say, AT&T's.

The benefit is that Google Voice will transcribe voicemail messages as text automatically, making voicemail messages more easily archived and searched. Other Google Voice features, like low-cost international calling and custom voicemail greetings, are available using an existing number too.

It's not quite number porting -- transferring a phone number from one telecom carrier to Google -- which brings with it the full Google Voice feature set -- the umbrella phone number for all one's phones, the ability to receive SMS messages via e-mail, and call blocking.

But Paquet believes conditional call forwarding may be enough for many users. "Potentially, a lot of people who felt they needed porting may be perfectly happy with this solution," he said. "As people come to understand all the benefits of the unique Google Voice number, we'll try to offer the opportunity to port."

Some carriers disable conditional call forwarding with pre-paid plans, so some Google Voice users may not be able to take advantage of these new changes to the service.

Paquet didn't have any information to provide about when Google Voice might open to the public. At the moment, the service is invitation-only.

Blue Cross of Northeast Pennsylvania, the University of Louisville School of Medicine, and a range of large and small healthcare providers are using mobile apps to improve care and help patients manage their health. Find out how. Download the report here (registration required).