And those agencies are very happy to assist, since they get a % for their work, and when the money is big. Most of they time it's very productive. Letters and calls are just the first step. A debt of 300$ can get to x000 pretty fast (debt, fees, lawyers). And then they go to court, put financial blocking on your assets, there is a judiciary person in charge of evaluating your assets, bank accounts, they can even freeze your debt from your bank accounts if you are refusing to pay a legal debt.
If you are abroad they will happily wait for you when you will get back, or get in touch via one of their foreign agency and let you know they are near.
If they would really want to get you in trouble they can inform the foreign local authorities about your problems, so depending on the case they can create problems for you.