This is the quintessential "lean on customer" letter from AT&T that is intended to put the fear of God into them so that they'll comply.  After all, it would be a shame if something were to happen to this nice business of theirs, wouldn't it?

For a monthly fee, AT&T will offer protection.  Of course, AT&T will benefit either way since they'll still bill the customer for fraudulent calls.  And, since the customer probably got their phone system from another company, AT&T won't be interested in any excuses...