The technology behind Voice over Internet Protocol (VoIP) phone systems hasn't evolved sufficiently to complete properly with landlines or mobiles, the report for Which? magazine found.

It looked at six of the most popular services, including Skype and Google Talk. Although the selling point of VoIP is the free calls, many of the systems cost to call people who aren't on the same network.

In addition, the systems can't call the UK emergency services.