adding text in one field and automatically to another

[william humphrey]

And now for something very easy.

I have two fields (area code and telephone number). I'd like for the first
field (area code) to allow three numbers to be entered and then if a fourth
number is entered automatically go to the telephone number field and
continue adding the rest of the number there.

I managed to limit the field to three numbers and go to the next field but
the fourth number always gets lost and has to be entered again.

Can someone suggest some code for this?  THanks....