A little bit yes. (See comments below) The app will make this seamless but should still use a bit of data.
Edit: After checking Google Help I found this:
By calling your own Google number
When you call your own Google number
and sign into the Google voicemail
system, you’ll have the option to
place a call. Just press 2 and enter
For calls within the U.S, enter area
code and number. For example:
212-555-2222. For international calls,
enter 011, the country code, and the
number (or "1 and the country code,"
depending on where you're calling).
For example: 011442012345612345 or