You can disable external follow me, it's defined in CDIV parameter in KSEXI, KSCAC, EXTEI commands or You can disable it in extension categories - EXCAC,
in KSEXI, KSCAC commands (digital extensions),CDIV parameter - the second bit must be set to 0.
in EXTEI (for analogue extensions), CDIV parameter - the third bit must be set to 0.
in EXCAC (for categories) , CDIV parameter - the thirs bit must be set to 0.