I don't know what is your motivation behind this question and what good will come from providing you with an answer. What I will say is that a significant number of VOIP providers all free calls between their account holders. So, technically, you and your family members could get a free account with say Callcentric and be able to place unlimited number of free calls between each other.
Apparently, the cost associated with in network calls are not significant enough to be an issue for OBihai or any of these providers. If you want a more definitive answer, why not drop OBiHAI an email with your question.