| BaseVoice | | Commercial software development company that produces PBX, router, proxy and application server software. |
| Brekeke Software | | Products for small to medium sized offices and a server database. |
| eSMS.com | | Provides UK phone numbers which forward calls to SIP. |
| Free IP Call | | A proxy service provider with information in English and French. |
| LignUp Corp | | Applications developer. Products include software PBX's for enterprise and service providers. |
| Ninetel USA | | A USA based proxy service provider and supplier of hardware phones and routers. Site contains call rates and service details. |
| OpenSER | | A branch of the SIP Express Router project, offering an open-source SIP server. |
| Parrish Systems | | Sells off the shelf code and provides a software consultancy service. |
| PC Phone Review | | News and reviews of hardware, software and service providers in the United States. |
| pulver.com | | A collection of information on the protocol. Contains news, reports and an IP telephony job noticeboard. |
| Secure SIP Client | | An RFC3261 client running on Win32 and WinCE PDAs supporting voice, video, sRTP and signaling security by TLS. |
| SIP Compose Group | | A working group experimenting and documenting the protocol. Contains various internet technical resources. |
| SIP Forum | | A membership based forum. Also contains a news board. |
| SIP Made Easy | | An introductory tutorial talks about protocol functions, components, message types, how it works and where it is applied. |
| SIP Wiki | | Wiki based resource containing information and links. |
| Sipcall | | A UK based service provider. Also contains hardware related links and protocol white-papers. |
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