voip app android,sip - Build a VoIP app in android - Stack Overflow

In simple words domain is the address of your sip server or an ip of the machine where your Sip server is running and sip server listens on 5060 and 5061 port by default .

There are many openSource sip server that are available on the internet that you can use .For example

opensips can act as Sip registrar,proxy server

kamailio can act as Sip registrar,proxy server

FreeSwitch can act as Sip registrar,A/V conference server,proxy server,IVR a complete PBX

asterisk can act as Sip registrar,proxy server,audio conference server,IVR a complete PBX