Are there vendors available who can provide entire solution including supply of gateways, gatekeepers, proxies, IP phones and accounting software? What could be the acceptable range of incurred costs to set up this network under an ISP infrastructure?
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It seems that you are considering providing IP Centrex services as an Internet Telephony Service Provider (ITSP). There are multiple suppliers that have products which combine many of the functions you have listed. One such supplier is Sylantro (http://www.sylantro.com). ITSPs going after enterprise business also typically offer a solution that is not hosted, but which is interoperable with their network. This allows them to provide service to a company that likes to have the IP PBX on their premises. The MX250 from Zultys (http://www.zultys.com) is a SIP based communication system for the enterprise which integrates many of the functions you mentioned. Regarding cost, it is very difficult to answer. It highly depends upon the range of services the ITSP is planning to offer.
This was first published in February 2004