>no replacement with existing carrier services etc.

If you mean the actual data <-> phone connection, then probably correct if you are frequently outside of wifi range. If you mean calling / texting, then I have used a google voice (sms) and skype number (calling) (they are the same number as gv forwards to the skype number for calls) for about 4 years now with no actual phone service. It's great because it works on almost any platform (even windows phone with sms to email forwarding, even though google doesn't support wp) and doesn't tie me to a single device like e.g. whatsapp.