Collection agents aren't how you want to resolve this. Instead you have an opportunity to solidify a relationship and create opportunity by showing you're serious about what you do, and you are worth being paid. Threatening people is hardly ever the right way to accomplish any of your goals

One of the best bosses I know in a rather small manufacturing company, after not being paid for 9 months, called up the CEO of the indebted company and said, "Hey Steve. I need directions from the airport to your office and then from your office to the nearest golf course, because Monday I'm going to fly out there personally, pick up this check, and then go play some golf. Feel free to join me if you want." Not only did he get his money, but the other company realized they were dealing with someone that was going to pursue every issue, no matter how small, and they started ordering far more product.

I'm not saying the exact same approach will work for you (though it could!) -- I'm saying get creative and get serious, and prove it.

His team was already set up to effectively pursue late payments. Less than 1% of their invoices make it past the 90 day mark.