1) Add-on no 2) Antithesis of int 3) Business card info 4) Business no 5) Business phone number add-on 6) Business phone number part 7) Business-card letters 8) Call routing no 9) Certain phone number 10) Contact info spec 11) Fire department device 12) In-house # 13) Inside no 14) Inside number 15) Internal phone no Found on https://www.crosswordclues.com/clue/ext
Ext was an elaborate extension of the minix computer filesystem. It had been completely superseded by the second version of the extended filesystem (ext2) by the late 1990's. Found on http://www.probertencyclopaedia.com/browse/GE.HTM