An FXO module is an interface. It can be used for dial in/out in addition to voice communications. If you are going to convert the analog t VoIP, then you need a Digital Sound Processor (DSP) for each concurrent voice call. So if you have an FXO VWIC, you will need 2 DSP modules to to use both of them. If you only use one for voice and one for something else, then you only need 1 DSP. If you have a T1 (24 channels), then you need 24 DSPs to use the whole thing. In addition, you can use DSPs for a few other things also.
Transcoder - Changing one compressin type to another. If you deploy Cisco Conference Connection then it only support G711. If you have sites that use G729 for your cisco phones, then you will need transcoder resources to convert the G729 to G711.
Conferencing - You can use DSP resources to act as a conferenc bridge. If you deploy callmanager express, then this is really your only choice. If you deploy callmanager on an Win2K server, then it can do some conferencing. I usually suggest that you have at least 6 DSPs more than you think you need. DSP resources are a bit expensive depending on the platform you are using, but are vital for those "must have" features that come up after the initial deployment.
So to reiterate the above, you n eed 2 thhings. A method for calls to come in and a DSPs to encode the voice traffic to IP.
It is what it is!!
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A+, Net+, I-Net+, Certified Web Master, MCP, MCSA, MCSE, CCNA, CCDA, and few others (I got bored one day)