Few things that you need to do
SIP applications are basically those applications using which you can call to mobile numbers through a network
You need to do a little research about SIP calling
You can find lots of free SIP calling credential from Zoiper for testing
Below are the understanding of work for which you need to provide estimations
Search for all IP address on a specific port on a network connected.
When we get all IP addresses on that port than we need to communicate on that IP address to register for SIP functionalities.
Need to send information about all IP addresses that we get and status of registering information using an API that we will provide you.
Need to redesign SIP application from sample application available at VAXVOIP, you need to download MS WINDOWS OS under SIP DEVELOPMENT KIT from given link
In case if you find difficulties with above library you can also take a look at PORTSIP