Voice over IP/Moving Away from Traditional Telephony
There are many compelling reasons for individuals and businesses to invest in Voice over IP. For smaller businesses or home users who frequently make long distance or timed phone calls, to larger commercial ventures looking to reduce costs on site-to-site telephony.
The range of VoIP plans available also allow more flexibility and cost control. Because VoIP termination does not involve any physical rental or consume resources when idle, plans are available on a per-use or prepaid basis, on a monthly subscription basis, or in any number of other configurations.
A successful VoIP deployment requires good planning and a good understanding of the strengths and weaknesses of IP Telephony. Using the right hardware, the right VoIP provider(s), and a customised dialplan can save significant amounts of money.
Both the positive and negative aspects of VoIP technology are detailed in the following sections.
Benefits of VoIP 
Cheaper long-distance - Because VoIP calls can be terminated anywhere in the world, long-distance calls can be made for much cheaper rates. For example, if you are a European VoIP user who frequently calls US numbers, you can purchase VoIP termination credit from a US VoIP provider, which will generally offer near local call prices from anywhere in the world.
Incoming Calls - For the same reason as above, VoIP provides the ability to obtain incoming telephone numbers from anywhere around the world, routed to you over the Internet. It's possible to obtain US 1-800 numbers, for example, no matter what your location.
No copper rental - Without the need for a circuit from your residence or business to your telephone company's exchange, your monthly costs can reduce significantly. Yet, there must be some means of connectivity from the "End User A" who initiates a call onto the Internet. Ostensibly, this connection is either a broadband connection (Cable or Telco DSL), or a "dial-up" connection. In both cases, and in this sense, obviously a copper rental is required.
Dial-Plan freedom - Using a dial plan, you have the ability to direct your softphone, hardware phone or VoIP gateway to send calls to different VoIP gateways or via land-line, depending on the number dialled. You also have the ability to set priority numbers, directing the gateway to drop an existing call in favour of the priority call where no lines are available.
Free IP-to-IP Calls - Where both the caller and recipient are using VoIP to place/receive calls, and where the correct configuration exists (see the section on ENUM at the end of this book), calls can be placed entirely over the Internet without any charge.
VoIP Issues 
As with any new technology, Voice over IP has its share of issues which must be considered carefully before implementation.
Emergency and Information services - There is no guarantee that calls terminated over IP for numbers such as 911 and 411 (in the United States) will be correctly routed. Due to the lack of location-specific termination of Voice over IP calls, it is not possible for such services to determine your location, and it is possible that the gateway your calls are terminating on does not have access to such numbers at all.
This limitation, and suggestions for workarounds, are discussed later in this book.
Bandwidth Issues - VoIP protocols can be very sensitive to both bandwidth limitations and network latency. Generally, VoIP calls must be placed via a broadband connection. Where there is heavy bandwidth utilization, high network latency, or the gateway is a significant distance from the user, call quality can be heavily degraded with the possibility that there will be no VoIP service at all.
See the chapter entitled traffic shaping and bandwidth control later in this book for suggestions on how this can be controlled.
NAT/PAT Issues - NAT (Network Address Translation) and PAT (Port Address Translation) are used to provide connectivity to machines in many situations such as private networks which require Internet connectivity.
With NAT, a gateway/router system will translate an IP address from the source address provided by the machine into another IP address entirely. NAT is often confused with PAT; however NAT provides a 1:1 relationship, eg. 192.168.10.10 will be translated to 184.108.40.206.
PAT is a slightly different translation. With PAT, one or more public IP addresses exist for any number of private/translated addresses. In the most common setup, a home user may have a residential DSL connection and a single public IP address, however they may have several computers on a private network behind their gateway/router which require Internet connectivity.
With PAT, the router/gateway will translate all private IP addresses into the public address provided, and will keep track of which ports established via this address belong to which private IP addresses, allowing the machines to share the public IP address.
VoIP protocols often have inherent issues with NAT and PAT. Due to the prevalence of these gateway devices for cable, wireless and DSL users, even users with single machines may find that PAT is performed by default via their router.
Please see the section immediately below entitled VoIP Protocols for more information on which protocols in particular are affected by NAT and PAT and how to avoid problems.