PSTN Access Prefix
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The PSTN prefix is defined on a country basis. It is specified for each service
provider for each country and each customer in the country. When the caller dials
a PSTN number with a PSTN access prefix (typically a 9 in the United Kingdom and
United States), this tells the dial plan that the caller is making an off-net call.

When the caller dials the PSTN breakout number, the dial plan routes the call to
the correct PSTN breakout location, whether it is a central or a local gateway.