According to the EQO.com web site: One way to think of EQO Mobile is that it turns your international calls into local calls:
Like any local mobile call, your calls on EQO will use your existing mobile plan's voice minutes (if you already have a plan with a lot of minutes, or unlimited evenings and weekends, you can take advantage of that airtime by using EQO and your existing minutes to make international calls).
The EQO network uses the Internet to help connect your calls, meaning your calls, text messages and IM messages will use a small portion of your existing mobile operator's data plan. It works best if the cell phone includes a data transfer plan.
The downsides: Some models of cellphones are not able to use this system yet.
I have heard great things from friends in Seattle who use this phone in Spain, including Tony who lives in Vejer de La Frontera