SIP and HTTP Servlets must not block threads in the body of a SIP method because the call state remains locked while the method is invoked. For example, Servlet method must not actively wait for data to be retrieved or written before returning control to the SIP Servlet container.

Servlets should also avoid sleep or wait operations in the body of a SIP method as such operations block the container thread and the callstate lock is held for the duration of each operation. Methods need to be written in such a way that control is returned back to the container and the flow is not compromised.