2003 was a landmark year for mobile communications in the United States. No, it was not the year Apple released the iPhone (News - Alert), but rather, the year when number portability between mobile operators was mandated.

Prior to number portability, changing carriers meant changing numbers, a painful process that prevented many users from switching. The pain of getting a new number was good for carriers as a churn reduction tool, but it was bad for consumers. The arrival of number portability meant users could more easily choose between competing carriers.

Some VoIP operators also took advantage of porting. Vonage (News - Alert) was among the first to do so, and a handful of others have done so since then. However, porting remains complex; it requires commitment to the target service provider. It takes time, and it requires users to go through a multi-step process. In recent years, new models have emerged that enable users to use their existing phone numbers with VoIP service providers, yet do not require porting – a not-port. How is that possible?

Broadly speaking, solutions for the not-port can be broken into two categories. The first category involves integration with the user’s existing carrier (carrier integrated), and the second category is done without any integration with the carrier at all (over-the-top).

The best example to date of a carrier-integrated not-port is Google (News - Alert) Voice. In March of 2011, Google Voice announced a partnership with Sprint, whereby Google voice users can use their existing Sprint numbers without porting to Google.

Carrier integrated not-ports involve two parties – an operator and an application provider. Calls to and from a user’s device are routed from the operator, toward the application provider, which can then process them (for example, by routing them to additional devices, as in the case of Google Voice), and then hand them back to the operator for completion of the call. In a sense, the call is hair-pinned from the operator to the application provider and back. Carrier integrated not-port solutions have the benefit of working for all calls to and from the user, without regard to whether the other party in a call is also using the same application provider. They also work without change to the user’s end device. However, they require bilateral arrangements with each individual operator, and such arrangements are complex both technically and financially.

Over-the-top solutions, on the other hand, can be done without involvement from any operator. To date, we’ve seen two distinct categories of over-the-top not-ports. The first category involves a mobile app on a user’s smartphone (mobile app), and the second category involves enterprises and uses software installed in their data centers (inter-domain). Good examples of mobile app not-ports are Apple’s Facetime, Tango and Viber.

The first time a person uses one of these apps, it verifies the user’s mobile number. Through this process, the application provider is able to determine, with a high degree of certainty, that the device on which the application is running is associated with a specific mobile phone number. Verification is typically done through SMS. The user enters in his or her mobile number, and the application provider sends an SMS to the phone which contains a code. The user copies this code into the mobile app, and the verification process is complete. The user is now identified by his or her mobile phone number. To receive a call by any other user of the service, these individuals need only be called using their existing mobile number. Indeed, for convenience, some of the mobile not-port apps scan the existing address book to find numbers that correspond to existing users of the service. Mobile not-port apps, unlike carrier integrated not-port solutions, only work for calls between users of the same application and require special software to be installed on the user’s device.

Inter-domain not-port solutions are quite different. Instead, they are enabled at the domain level with software deployed in data centers, and do not require software on the user’s device. They assume that the domain has obtained phone numbers from existing PSTN providers, and utilize VoIP to enable inter-domain calling without sending calls through the PSTN provider.

The best example to date of an inter-domain not-port solution is Cisco’s (News - Alert) Intercompany Media Engine, which uses Verification Involving PSTN Reachability technology. ViPR, like the mobile-app solutions, involves verification of numbers. Verification is a fundamental process that is shared by all over-the-top not-port solutions. However, ViPR verifies numbers through an invisible process that uses PSTN call detail records as a form of shared secret between domains.

Unlike mobile-app not-port solutions, inter-domain solutions work with whatever endpoint the user has – typically a hard phone in enterprise deployments. Inter-domain not-port solutions are also different in that they are by definition inter-domain, and require that the other domain has deployed compatible technology.

With these solutions now in the marketplace, customers are faced with a new choice: to port, or not-port – that is the question.

Jonathan Rosenberg is chief technology strategist at Skype (News - Alert) (www.skype.com).