How to communicate two device via one router offline using VOIP or something else?
I want to make communication stream between two iOS device using Home router. I have read about VOIP but did not find interesting.
You question is kind of vague. What do you mean one router offline?
If you wish to connect two device no matter if its cellular or wifi (you can specify either) you can use the Bonjour and NSNetservice Libraries.
From Apple: The Bonjour zero-configuration networking architecture provides support for publishing and discovering TCP/IP-based services on a local area or wide area network.
It's zero config and easily implemented. I think it's exactly what you are looking for.