A soft phone is a software program that provides telephone functionality on a non-telephone device, such as a PC or PDA. So how do we recognize such a beast? What might at first glance seem a simple question actually raises many. A soft phone should probably have some sort of dial pad, and it should provide an interface that reminds users of a telephone. But will this always be the case?

The term "soft phone" can be expected to evolve rapidly, as our concept of what exactly a telephone is undergoes a revolutionary metamorphosis.^[40] As an example of this evolution, consider the following: would we correctly define popular communication programs such as Instant Messenger as soft phones ? IM provides the ability to initiate and receive standards-based VoIP connections. Does this not qualify it as a soft phone? Answering that question requires knowledge of the future that we do not yet possess. Suffice it to say that while at this point in time, soft phones are expected to look and sound like traditional phones, that conception is likely to change in the very near future.

As standards evolve and we move away from the traditional telephone and toward a multimedia communications culture, the line between soft phones and physical telephones will become blurred indeed. For example, we might purchase a communications terminal to serve as a telephone, and install a soft phone program onto it to provide the functions we desire.

Having thus muddied the waters, the best we can do at this point is to define what the term "soft phone" will refer to in relation to this book, with the understanding that the meaning of the term can be expected to undergo a massive change over the next few years. For our purposes, we will define a soft phone: any device that runs on a personal computer, presents the look and feel of a telephone, and provides as its primary function the ability to make and receive full-duplex audio communications (formerly known as "phone calls")^[41] through E.164 addressing.^[42]