This is by all means not any sort of legal advice, this is just my personal opinion from working in the collection industry.
By law, they are not allowed to threaten you with legal action unless they actually will proceed with that action. If they do proceed with that, they first must send you a written notice advising you of this. However, this may be slightly different if its a law firm contacting you.
However, remember the statute of limitations. If you went into default before 2004, the limit is 6 years for the creditor to pursue you legally. If its after 2004, they have 2 years to pursue. However, they can still attempt to collect on the balance.
There are certain strategies you can use to limit communications with these organizations. However, I would only recommend them if you are willing to pay the debt as opposed to using the methods to run away from your debts.