If the number was saved by Firefox's "address autofill" feature -- this may turn the field yellow when a suggestion is filled -- try the steps in the following article to clean up your list (it tends to accumulate a lot of little variations over time):

Automatically fill in your address on web forms

If the number is not on that list, or the field doesn't turn yellow, it may be the "form history" feature (an older feature that could store anything). The next time you see it suggested in a form -- likely on a little drop-down below the field -- use the keyboard to arrow down to it and then press Shift+Delete to remove it. Those are Form History entries. More info on those:

Control whether Firefox automatically fills in forms

Was it either of those?

I have tried the "Shift Delete" and it accomplishes nothing, The wrong information still auto fills.

If I type in the proper phone number, the second I auto fill any other field, the phone number changes to the wrong one.

I fill forms out and would use a functioning auto-fill several times a day. Why does Firefox have an auto-fill option if they can't make it work?

It sounds like Address autofill, which stores multiple fields as a group. That was the first feature I mentioned. Did you check the Address autofill list on the Settings page to see whether you can remove the unwanted data there? That was the second section of the article I linked to.