Reliability

The existing PSTN telephone network is solid and reliable. Telephones work practically all the time and we have become very dependent on them. Computers, e-mail and other new techniques still aren't as reliable as the good old telephone. If we’re honest, there will be very few people who panic if they can't use their e-mail for an hour. We accept this, it’s part of the game. However, if the telephone line is dead, that is more inconvenient. PSTN is not as efficient as VoIP, but it is much more reliable. The Internet uses a network which is much more complex and as a result more vulnerable. Thus, reliability is one of the biggest shortcomings of VoIP.

First of all, VoIP depends on electricity that comes from a wall socket. A POTS telephone gets its power from the telephone line. In case of a power failure your telephone will keep on working (although a wireless phone depends on battery power). When it comes to VoIP, no power means no telephone. Therefore, a stable power supply is a must for VoIP.

Dependence

Moreover don't forget that, if you have an ADSL connection, it still depends on the analog telephone line. Without a telephone network, it would obviously be necessary to set up a network (Internet) independent of the existing telephone lines.

Emergency number

Today, if anyone calls an emergency number with his standard telephone the geographical position of the caller can be determined automatically. This is important in the event that the caller is not in a position to explain his or her location accurately. With only an IP address, it is much more difficult, if not impossible, to determine the location of the caller.

Stability

As we stated before, the Internet is not as stable as the telephone network. There is the possibility of packet loss. In the case of pure data transfer this is not a problem, the packets are re-sent. For telephone conversations, this is of course much more annoying, because a part of the conversation is omitted. Before switching over totally to VoIP, there must be a certain stability guarantee for the network.

Vulnerable

As well as other forms of digital data traffic, VoIP is also vulnerable to viruses, worms and hackers.  This all requires data encryption to protect the packets.

If you make phone calls using a softphone on your PC, you are dependent on the reliability and speed of your computer. If you computer crashes, you will lose your VoIP connection.