Yes you can! Check up this answer about Talkatone here, it is based on Google Voice. The sound -quality is not the best but I like the idea that I actually need just a WIFI to make cheap international calls.
I have actually replaced my landline/mobile/local -phones vith Google-Voice because it is much more convenient to pay to it through credit card and much more convient than carrying a massive amount of PrePaid/Post-paid -cards when travelling-or-just-living-well-and-free-etc, highly recommended tool indeed! Talkatone in iPad is a killer, a bit like FaceTime but integrated to Facebook and probaly to other services in the future. Talkatone in Android is similar, good integration so saving time in toning all contacts from Google Contacts, FB Contacts, etc. If you are a buz, you can imagine how much time this will save -- you need less to worry about crazy international -phone charges and you can actually drop out mobile-phone -contracts altogether, just charge employers with Google Wallet/Voice!
Talkatone has poor sound -quality because it wants to sell its Premium service but if ads and a bit poorer sound quality do not matter, it has worked for me, nicely (haven't tried the Premium -service). A good/bad side is that your number is some oddity but extremely good side is that you do not need to know the prefix with internatinal calls in using Google Voice, super convenient!
-Sim-card?
-Yes you don't need a SIM -card to make international calls, just WIFI. Phone does not require SIM -card, dot. Victory :)
