Hi, The UA application on a mobile device will be developed using WindowsCE.NET 4.2 and the SIP Proxy and Registrar Server will be a Linux box. Is there any interoperability issues that i need to know about. Experts please advice. Also, any tips on how to go about the implementation of a Stateless Proxy and Registrar Server on a Linux box will be of great help. I did download the ser free server from the website. Could anybody guide me as to where to start working with the package i obtained. I am new to this so please pardon my ignorance. I am keen on learning and improving. Please guide. Thanks.