From the knowledgebase:
When are Voice Compression Channels Used
The voice compression channels are used as follows. • IP Device to Non-IP Device
These calls require a voice compression channel for the duration of the call. If no channel is available, busy indication is returned to the caller.
• IP Device to IP Device
• Call progress tones (for example dial tone, secondary dial tone, etc) do not require voice compression channels with the following exceptions:
• Short code confirmation, ARS camp on and account code entry tones require a voice compression channel.
• Devices using G723 require a voice compression channel for all tones except call waiting.
• When a call is connected:
• If the IP devices use the same audio codec no voice compression channel is used.
• If the devices use differing audio codecs, a voice compression channel is required for each.
• Non-IP Device to Non-IP Device
No voice compression channels are required except for Small Office Edition Embedded Voicemail access.
• Music on Hold
This is provided from the IP Office's TDM bus and therefore requires a voice compression channel when played to an IP device.
• Conference Resources and IP Devices
Conferencing resources are managed by the conference chip which is on the IP Office's TDM bus. Therefore, a voice compression channel is required for each IP device involved in a conference. This includes services that use conference resources such as call listen, intrusion, call recording and silent monitoring.
• Page Calls to IP Device
Page calls require 1 voice compression channel per audio codec being used by any IP devices involved. IP Office 4.0 and higher only uses G729a for page calls, therefore only requiring one channel but also only supporting pages to G729a capable devices.
• Voicemail Services and IP Devices
Calls to the IP Office voicemail servers are treated as data calls from the TDM bus. Therefore calls from an IP device to voicemail require a voice compression channel.
• On the Small Office Edition, embedded voicemail uses voice compression channels for audio conversion. Therefore all calls to Small Office Edition embedded voicemail require a voice compression channel and calls from IP devices require two voice compression channels.
• Fax Calls
These are voice calls but with a slightly wider frequency range than spoken voice calls. IP Office only supports fax across IP between IP Office systems with the Fax Transport option selected. It does not currently support T38.
• SIP Calls
• SIP Line Call to/from Non-IP Devices
Voice compression channel required.
• Outgoing SIP Line Call from IP Device
No voice compression channel required.
• Incoming SIP Line Call to IP Device
Voice compression channel reserved until call connected.
• T38 Fax Calls
IP Office Release 5+ supports T38 fax on SIP trunks and SIP extensions. Each T38 fax call uses a VCM channel.
• Within a Small Community Network, an T38 fax call can be converted to a call across across an H323 SCN lines using the IP Office Fax Transport Support protocol. This conversion uses 2 VCM channels.
• In order use T38 Fax connection, the Equipment Classification of an analog extension connected to a fax machine can be set Fax Machine. Additionally, a new short code feature Dial Fax is available.
Note: T3 IP devices must be configured to 20ms packet size for the above conditions to apply. If left configured for 10ms packet size, a voice compression channel is needed for all tones and for non-direct media calls.
Measuring Channel Usage
The IP Office System Status Application can be used to display voice compression channel usage. Within the Resources section it displays the number of channel in use. It also displays how often there have been insufficient channels available and the last time such an event occurred.
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