A Dial Plan is a specially crafted text string, or script, that specifies how to interpret digit sequences as dialled by the VoIP user and how to convert those digit sequences into an outbound dial string to be used by your VoIP service provider for call routing and termination.
A dial plan will look something like this:
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I like to think of a dial plan as a type of "filter" that compares dialled digit sequences against the "rules" that have been specified in the dial plan script. If the dialled digit sequence match any preset pattern or rule specified in your phone's dial plan script, your phone (ATA, IP-Phone, softphone, etc) will proceed to transmit the "validated" dial string to your VoIP service provider for call processing.
If the digits you dial do not match or pass validation by the predefined dial string script, the dial string is rejected and will not be sent to your VoIP service provider.
Generally speaking, the dial plan protects you from sending invalid dialled digit sequences to your VoIP service provider. A dial plan can also be used to restrict calls to specified destinations. As well, a dial plan can be crafted to prepend, append, or substitute the digits you enter into your keypad.
Reasons for using dial plans:
- Ensures that numbers are correctly dialled.
- Allows various methods of speed-dialling.
- Can be used to only allow calls to specific geographic locations.
- Can be used to block numbers to specified destinations.
On the Cisco Linksys SPA series of phones, each outgoing line will be governed by a separately defined dial plan. This is because each line may be registered to a different VoIP service provider. Different VoIP service provider's may handle dial sequences differently or have different special dial string sequences for special call processing. Thus, it is necessary to be able to customize each dial plan for each VoIP service provider being used.