To dial out on a trunk, 4K must find a digit match in WABE as CO or TIE. You can't use the same digits for CO or TIE as you have for STN. So as eniyamath says, any digits dialled can't also be used for extensions.
If you just want one extension to do this (example, phone is used by the public and you don't want them to dial prefix code), then give the phone a hotline class of service, have it hotline the CO code, and set ZAND CODTN to yes, so that 4K generates CO dialtone to the user, after receipt of a CO code (which hotline automatically dials when the phone goes off hook).