Disadvantage of VoIP

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VoIP technology still has a few shortcomings that have led some to believe that it is not ready for widespread deployment.

Faxes

One drawback is the difficulty in sending faxes due to software and networking restraints in many systems. However, an effort is underway to define an alternate IP-based solution for delivering Fax-over-IP, namely the T.38 protocol. Another possible solution to overcome the drawback is to treat the fax system as a message switching system, which does not need real time data transmission. The end system can buffer the incoming fax data before displaying or printing the fax image.

Internet Connection

Another disadvantage of VoIP service is its reliance upon another separate service - an internet connection. The quality and overall reliability of the phone connection is entirely reliant upon the quality, reliability, and speed of the internet connection which it is using. Shortcomings with internet connections and Internet Service Providers (ISPs) can cause a lot of grief with VoIP calls. Higher overall network latencies can lead to significantly reduced call quality and cause certain problems such as echoing.

A few business VoIP Providers overcome this challenge using dedicated connections (point to point T-1s) between a client location and the VoIP Providers gateway facility where VoIP is converted back to traditional local phone service. Using a point to point connection with specialized routers (which prioritize packets), the highest quality of service can be achieved.

Many VoIP users still maintain a traditonal analog voice line (business line) which allows them to utilize a traditional fax machine when needed and can also be used to call 911 service if you have an analog phone on the fax machine as well.

Power Outages

Another disadvantage of VoIP is the inability to make phone calls during a power outage, but this problem also exists with many phones used with conventional land lines, and can be remedied with a battery backup. During a power outage, you also have the choice to forward your phone to your cell phone or another phone number so you can still receive calls. Although you can't call out on your phone system during a power outage, at least you can still receive calls.

If VoIP is used in solitary LAN (with no internet connection), it would consume more resources compared to a PABX.

Modems are now Avaliable with lithium ion battery backup so that you can use the service with no power.

Implementation challenges

Because IP does not provide a mechanism to ensure that data packets are delivered in sequential order, or provide Quality of Service guarantees, VoIP implementations face problems dealing with latency and jitter. This is especially true with satellite circuits, due to long round trip propagation delay (400 milliseconds to 600 milliseconds for geostationary satellite). The receiving node must restructure IP packets that may be out of order, delayed or missing, while ensuring that the audio stream maintains a proper time consistency. This functionality is usually accomplished by means of a jitter buffer.

Another disadvantage of VoIP is routing traffic through firewalls and address translators. Private Session Border Controllers are used along with firewalls to enable VoIP calls to and from a protected enterprise network. Skype uses a proprietary protocol to route calls through other Skype peers on the network, allowing it to traverse symmetric NATs and firewalls. Other methods to traverse firewalls involve using protocols such as STUN or ICE.

Reliability

Conventional telephones are connected directly to telephone company phone lines, which in the event of a power failure are kept functioning by back-up generators or batteries located at the telephone central office.  Even with local power still available, the broadband carrier itself may experience outages as well. While the PSTN has been matured over decades and is typically extremely reliable, most broadband networks are less than 10 years old, and even the best are still subject to intermittent outages.

Quality of Service

Another potential disadvantage of VoIP is Quality of Service. Where IP packets are lost or delayed at any point in the network between VoIP users, there will be a momentary drop-out of voice. This is more noticeable in highly congested networks and/or where there are long distances and/or inter-networking between end points. Technology has improved the reliability and voice quality over time and will continue to improve VoIP performance as time goes on.

Emergency calls

What some consider to be the biggest disadvantage of VoIP is emergency call handling.  The nature of IP makes it difficult to geographically locate network users. Emergency calls, therefore, cannot easily be routed to a nearby call center, and are impossible on some VoIP systems. Moreover, in the event that the caller is unable to give an address, emergency services may be unable to locate them in any other way. Following the lead of mobile phone operators, several VoIP carriers are already implementing a technical work-around. For instance, one large VoIP carrier requires the registration of the physical address where the VoIP line will be used. When you dial the emergency number for your country, they will route it to the appropriate local system. They also maintain their own emergency call center that will take non-routable emergency calls (made, for example, from a software based service that is not tied to any particular physical location) and then manually route your call once learning your physical location.

The United States government had set a deadline, requiring VoIP carriers to implement E911; however, the deadline is being appealed by several of the leading VoIP companies.

This is a different situation with IPBX systems, where these corporate systems often have full E911 capabilities built into the system.

Integration into global telephone number system

While the traditional Plain Old Telephone System (POTS) and mobile phone networks share a common global standard (E.164) which allocates and identifies any specific telephone line, there is no widely adopted similar standard for VoIP networks. Some allocate an E.164 number which can be used for VoIP as well as incoming/external calls. However, there are often different, incompatible schemes when calling between VoIP providers which use provider specific short codes.

Caller ID

Caller ID support among VoIP providers varies. When calling a PSTN number from some VoIP providers, Caller ID isn't supported, and the target person will not know who is calling. The number shows up as 'Unknown' or '000-012-3456'.

But the majority of VoIP providers now offer full Caller ID w/ Name on Outgoing calls.

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