Google's introduction of free phone calls within Gmail has proved popular, with Google announcing on Twitter that 1 million calls had been made in its first 24 hours.

The new feature, released Wednesday, is accessible in a link that has popped up in the chat interface for Google's Gmail. The free digital phone service is potentially upsetting for the market held by Skype and other Voice Over Internet Protocol phone services that charge a small fee. Phone calls can be made and received for free, but international calls require a relatively low fee, according to Google's own blog post on the feature.

Initial reviews of the service say connection quality is as good as Skype's but that there are a few drawbacks, including a lack of service for other countries. It also requires a fairly modern operating system for the voice and video chat plug-in that must be downloaded and installed, either Linux or Windows XP or MAC OS X 10.4.