First, there might be a very low cost or even free "basic" or "introductory" tier that's just so feature poor that the vast majority of customers will opt for the next level up, which will be the full-priced tier. Another common practice is a one- or two-year contract, each with a slightly lower price that are offered next to a significantly higher-priced month-to-month tier. Additionally, while most residential VoIP services offer unlimited calling, some vary their pricing on call restrictions. Those will come either in the form of minutes (with higher pricing attached to monthly overages) or geographic region. The latter usually start with nationwide calling and then tack on another charge for worldwide calling or even separate charges for different countries.
VoIP service simply enables your phone calls to be transmitted and received over your home Internet connection instead of your telephone landline. This is perhaps why some people refer to the service as being an Internet phone service or broadband phone service. This does mean though that you need a high speed Internet service to be able to use VoIP for your phone service.