Received a mail from a collections agency
Here are some educated guesses:  B has a collection agency which they regularly use.  They sell their bad debts to this agency at a (deep) discount and now that agency owns the debt.  They're taking a gamble that they can collect the debt.  (It is very possible that this "separate" collection agency is really just an office on campus.  But that makes little difference.)

I would write the collection agency and tell them that I have never set foot on B's campus and that I don't owe any money to them.  I would make a sentence like, "As an international student, I could not be enrolled at an American university without an i20.  If B cannot produce my i20, that is proof that I never attended there."  I would guess that at this point the agency would realize they lost their bet and go on to the next victim.  

If not, things have entered the "nasty" stage.  I doubt it gets so far, but if necessary, write a letter to B accusing them of fraud.  Your premise is that they know that foreign students are typically naive and apply to many American schools.  If they find that a student applies at B and but attends another American school, then they send a bill hoping the student is afraid of running afoul of obscure American laws.  They use scare tactics to try to collect tuition and fees to which they are not entitled.  Fearful of being deported, many of the foreign students pay the bill.   Something like that will at least get their attention and a human will check the records carefully before they make another move.   The last thing a university wants is an investigation.  At the moment, some bored peon is glancing at your letters or day-dreaming during your phone calls.  You need to get them to sit up and pay attention, and such a letter will do that.  If they look at the record seriously, that should be the end of it.

If not, send the same letter to the local county attorney and/or the district attorney.

Let me repeat that I'm just guessing my way through this given the data at hand.