There is no SPN involved in this type of dialing. But first, are you in the USA?
If you want users to dial 4 digits, you create Steering Codes (DSCs). The Steering Code points to an RLI. The RLI points to the trunk route.
The SPN does not come into play in this. The Route List holds the DMI which inserts the (for example) first 3 digits or first 7 digits 1NPANXX. The steering code would be the same as the extension in some cases.
If for example, all the extensions on the distant system started with 4, then the steering code could be 4. ("could" being the key word). Or they might be 41, 42, 43, 43, or it might be 402, 403, or it might be every DN, 4020, 4021, 4022, 4023, etc.
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Nortel Resources at GHTROUT.com
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