That all comes down to your dial-peer configuration. There is a reason why most PBX systems use an off-net code: to distinguish between internal extensions and off-net patterns. If you have four-digit extensions in the 2XXX range and dial a PSTN number starting with 2T you will have a problem; IOS will match the internal pattern first.
Look at your dial-peers, specifically the destination-pattern sub-command. It should start with 9 today. You can remove the 9 and just have whatever comes after it such as 67T. Just remember that any explicitly-matched digits are stripped. If you match 67T then you would need to prefix 67 to get the full number sent out to the PSTN.
Be very careful not to create overlaps here, as Jonathan stated there is a reason most companies use off-net access code, once you get rid of the off-net access code your options for internal extensions are limited depending on what country/state/city your deployments are in and how you dial PSTN.
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Please find the attached HTML document, download and open it on your PC.
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