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There are a few different ways of manipulating the caller id that is sent to a given gateway; each of these apply to any call sent out through that gateway. Normally, any of these settings may be overridden by settings that are specific to a particular user as described xecsuserV4r2:below; to prevent a user-specified value from overriding the gateway value, check the 'Ignore user Caller ID' checkbox.
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If you want all outgoing calls to have the same caller id (typically true if all or most users do not have a DID so incoming calls go to a single 'main' number), then you can associate the number you want with the gateway.
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If you have users who have individual Direct Inward Dial (DID) numbers, and those users' extensions can be mapped to the DID numbers using some pattern, then you can configure the pattern once at the gateway. To do this transformation, check the 'Transform extension' checkbox, and then set the following two fields:
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