NkSIP is an Erlang SIP framework or application server, which greatly facilitates the development of robust and scalable server-side SIP applications like proxy, registrar, redirect or outbound servers, B2BUAs, SBCs or load generators. NkSIP takes care of much of the SIP complexity, while allowing full access to requests and responses. A single NkSIP instance can start any number of SIP Services, each one listening on a specific set of transports (udp, tcp, tls, sctp, ws or wss), ip addresses and ports. A Service can provide a callback module where it can implement a number of callback functions. All of them are optional, having sane defaults in case they are not implemented. For outgoing-only SIP applications, a callback module is not necessary.