Originally Posted by One-X-User
Yes, I am on a Go-Phone plan. But the phone is unlocked so I can use it on that plan. So I think that is the reason I cant get OTA updates.
Go Phone is still AT&T, so this is not the reason you are not getting the update.
There can be several reasons for not getting the update. Some people have had success doing a factory reset (backup your personal data first), and then the update will go through. Or since you missed the 2.20 OTA, this seems to break the OTA sequence (you need 2.20 OTA before the 3.18). I understand this technically shouldn't happen, but I've seen it plenty. You might be able to ask AT&T to push the 2.20 OTA to you. But they might not support you, since the phone is not originally intended to be on a prepaid plan (not sure on the exact rules of this).
But the easiest way to update to 3.18 is to RUU like timmaaa said. Its a cleaner install then OTA, anyway (no need to do 2.20 RUU first). Although it will wipe the phone, so backup your personal data first.
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Phone: HTC One X (AT&T version) Rooted, Bootloader Unlocked, S-Off
Tablet: Samsung Galaxy Note 8 (WiFi only) Rooted