[jQuery] [validate] callback onblur on single input field

hi
i'm using tha validation plugin on a very long form. It works fine
everywhere but for one little issues:
i need to trigger a function onblur of a single field: i got a "tel"
and a "mobile" input field one of which is required (at least).
How i trigger the callback after i fill the input to erase th error
class from the other field?
Thank Vitto