In SimpleProxyServlet, you override several methods to respond to the main SIP methods.
doInvite- responds to INVITE requests.
In SimpleProxyServlet, upon receiving an INVITE request the servlet will create a javax.servlet.sip.Proxy instance, set some options, and proxy the request to the target SIP server.
doBye- responds to BYTE requests.
In SimpleProxyServlet, the servlet logs a message upon receiving a BYE message, and calls the doBye method of the parent class (javax.servlet.sip.SipServlet).