Here are the list of Free VoIP Proxy that you can use with your VoIP System:
Partysip is an implementation of a SIP proxy server. SIP stands for the Session Initiation Protocol and is described by the rfc2543 (soon to be deprecated by latest revisions). SIP is a open standard replacement from IETF for H323.
Partysip is a modular application where some capabilities are added and removed through plugins. The program comes with several GPL plugins. At this step, partysip and its plugins could be used as a 'SIP registrar', a 'SIP redirect server' and a 'SIP statefull proxy server'. (stateless capabilities have been removed)
siproxd - SIP proxy/masquerading daemon
Siproxd is a proxy/masquerading daemon for the SIP protocol. It allows SIP clients (like kphone, linphone) to work behind an IP masquerading firewall or router.
Load Balancer Proxy
The Load Balancer is a very simple proxy that is useful in SIP-based VoIP installations where there are multiple ingress proxy servers. The Load Balancer permits pooling these servers, thereby eliminating the need to balance user demands for connectivity through a complicated provisioning algorithm.
All users can send their INVITEs and REGISTERs to the same SIP URI and the Load Balancer will assign ingress proxy servers dynamically to each transaction. In this way, the traffic load is balanced over a pool of proxy servers based on the real-time demand for services.
The STUN (Simple Traversal of UDP through NATs (Network Address Translation)) server is an implementation of the STUN protocol that enables STUN functionality in SIP-based systems. The STUN server tar ball also include a client API to enable STUN functionality in SIP endpoints. In addition there is a command line UNIX client and a graphical windows client that check what type of NAT the user is using.
STUN is an application-layer protocol that can determine the public IP and nature of a NAT device that sits between the STUN client and STUN server. The current version of the code supports most of RFC 3489 except the ability to get OTPs from the server.
Recommend the tutorial