Ext. is a written short form of extension when you mean one telephone line inside a bigger number. The main number reaches the company, school, or hotel; ext. 12 reaches one desk. You will see it on name cards, email footers, and office signs: 02-2555-1212 ext. 18. People almost always say “extension” out loud; they do not say “ext” as a word, and they only spell the letters if they are reading a form. Though actually, the same three letters can stand for other things in other kinds of notes — but sitting after a phone number, this is the only reading that fits.