Here's why:
* The "o" in "phone" has a short vowel sound. It sounds like the "o" in "hot" or "box".
* Long vowel sounds are usually spelled with a single vowel letter followed by a silent "e" at the end of the word. For example, "hope" or "cone".
So, while "phone" has an "o" in it, it's not a long "o" sound.