So... what is VoIP:

VoIP is Voice over Internet Protocol, or IP telephony. This is a technology that enables you to make ordinary telephone calls over a computer network or over the Internet. VoIP converts the data signals from your telephone into digital signals and sends them in packets over the Internet. The data is then converted back to telephone signals on the other end, allowing you to speak to virtually anyone.

Today, companies use VoIP technology to reduce or eliminate long-distance costs, include remote or home-based employees on their enterprise calling systems, or enable these remote employees to retain a company extension as if they were working on-site.