[SR-Users] SIP registration

Hello     I am devloping the chat/call application like viber,whatsapp etc in Android using sip. I want to know how i can register the sip account with ur sevrer. what's code which i need to write in my application.suppose if i want to chat or call between other person how i can get register without telling user that i am going for sip registration means there i cam not going to display sip registration form etc. where i can save user message and how i can maintain user info like dispaly name , display pic etc.is there any serverside databse involvement to save messages,pic etc or sip sever maintain automatically. Kindly describe in details as i am new in Android Thank you   Best Regards Mudasser Tufail Senior Software Engineer