It is a hired collection agency by one of your credit card companies. The new system they Capital One has for payment harrassment is that the company will fluctuates the area codes, as well as numbers to throw 'you' off from knowing who they are. That way they can call as many times as they like. It is a psychological tactic.
The reason why they will not harrass you via the Postal Service is because it then becomes a federal offense (the Post Office is a Government run edifice) and no one wants to go to jail for your nonpayment of a credit card.
It is the credit card company's business to give you a 250 dollar limit. When you charge up to your limit, that's when you find out that their late fees, userious rate of interst, and other fees on top of that 250 dollars have you owing the credit card company 800 dollars. That's a nice tactic, as well.
Anyway, when you don't pay up - you get the calls. With recession, people losing their jobs, homes, lives in war - it might be a bit more understandable if you see this as them doing their best to try to stay in business. Look around. A lot of companies are going bancrupt.