Text box for my Mobile/Cell Phone Device is missing in Settings

I'm unable to add my mobile/cell in my profile - the box where you are supposed to enter the phone number is not showing up on the /devices page? I've tried using FireFox and Safari - the box where you enter the phone number isn't there. I check the box saying we agree to SMS, and click save, and get an error saying you have to enter a valid phone number. But again, the box where you are supposed to be typing in the number isn't on the page - I've had several people look at it, and they don't see it either. I've got 3 other twitter accounts that work fine - but this one is not giving me the text box to type in my mobile device under Settings.