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If phone number is 02871 2397567 and you want  to let pass only last 3 digits (567) to extension 12, you have to cut off  02871 2397 and in "Inbound routes" set "Pattern" to ^(567) and "Target" to 12.

There are several ways forms how number can come from provider, depending on the provider. That's why you have to check all possibilities.

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                                      ^(?: ( ?: (?: 0049|\+49|49|0)?2871)? 2397 )?(.*)    567 will be saved in $1 and compared to the pattern in "Inbound routes".

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                        ^( ?: (?: 0049|\+49|49|0)?2871)?(.*)                      2397567 will be saved  in $1 and compared to the pattern in "Inbound routes".

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(.*) – all numbers which still are there

$1 – is variable, in this variable will be saved all from (.*)

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