Whenever gVoice stop the XMPP support, i.e. stopping gVoice API being public, we MAY need to find other way for free NA calls.
For people with VOIP device:
1. check device provider (an Obihai can always call another Obihai for free, @ OB has a unique #), or
2. SIP calling - e.g. http://www.callcentric.com/rate/plans/ip_freedom (Obihai may have this function, I can't remember)
For people with actual phone: (I'm only aware of the following worldwide free calls, messy)
1. Rebtel.com (only 10 seconds talk time, free if callee return call right away)
2. Jaxtr (similar to above but for mobile only, send SMS for callee to call back) (may work for landlines IF u can figure out the call back timing) https://voice.jaxtr.com/user/index.jsp