The Snom One Yellow Software-Based PBX is a SIP-based IP telephone system which is implemented in a back-to back user agent (or B2BUA), so all traffic between two endpoints traverses the system. This gives the system complete control of the call state and allows it to participate in all call requests. It also allows it to perform a number of functions that are impossible using the SIP proxy model. A few such functions are forking calls to the cell phone, returning them to the system, if needed and connecting phones remotely to the system. The B2BUA also facilitates advanced media features such as call recording, barge, listen in and whisper. The system is easily portable to most operating systems and requires only a small amount of memory. Doing database lookups can be slow in a very busy system. It is much faster to have a data structure in memory with the necessary information to authenticate a phone.