TekSIP is a SIP Registrar and Stateless SIP Proxy for Windows. You can use freeware version for both commercial and personal use. We only ask you to provide feedback.
TekSIP tested on Microsoft Windows XP and Windows 2003 server.
TekSIP complies with RFC 3261, RFC 3263, RFC 3311, RFC 3581 and RFC 3891. It supports NAT traversal, UPnP and ENUM. Please see technical details about NAT traversal in Readme.txt You can select IP address to be listened and alternative SIP endpoints for outgoing calls. You can also log session details into a log file and monitor active registrations and sessions in real-time.
TekSIP also supports UPnP IGD specification. If installed behind UPnP supported Internet gateway device (ADSL router e.g.), TekSIP automatically detects if it is behind a new NAT gateway and the external IP address. All outgoing requests manipulated for NAT traversal. You do not need to add manual reverse mappings for SIP and RTP protocols.
TekSIP can optionally act as a B2BUA for incoming 3xx SIP responses. RADIUS Accounting (RFC 2866) is supported.
TekSIP also provides single accounts proxy; if you have just one provider account and many internal clients, TekSIP proxies all external calls for the provider account.
TekSIP is free for both commercial and personal use.
Download: TekSIP 2.1