For a given task can give solution for cisk. This option is unlikely to be the cheapest though, I think one of the most stable in the work. And You do such a decision would not run, although, if necessary, will be able to help remotely. Yes, for free.
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To take, better used, the 2811 router ( 2801, but it is not fundamentally cheaper and noticeably weaker in the future). If you get voice bundle take — great, there will already be modules for recoding voices.
New clean 2811, the idea is that you can get for somewhere around $1400 plus remap module for about $500 (up to 16 streams)
How much you can get used, especially if there is a way into eBay, you will not tell.
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On it pour Call Manager Express (nothing complicated, even a few video instructions). License the fairness of this procedure, I did not specify :)
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This gateway to put in the Internet and for him to have IP inbound (and outbound) voice stream from operators. Traffic can be encrypted, if there is a demand and operator support. With online channel, you have to think — maybe we should take separate for that matter — depends on availability in Your particular case.
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If you take E1, then it is about $600 for the module interface to the router. But I don't see the point in taking 30 lines where enough for a dozen.
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Inside users to deliver software Communicator and issue the headset.
If necessary, raise the VPN gateway and put users anywhere — even at home.
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I repeat — this solution may not be the optimal cost, but it is guaranteed to work in the mode of "set it and forget it".