I am a bushiness owner in the financial and debt help services arena in Canada. I am a certified Debt Arbitrator and Credit Counseling Specialist.
We have been down the same road in the past for sure. Time and Time again. E-mail is a terrible strategy phone is always best. A great way to get a response is to text them if you have their mobile numbers. A great site for that is Twilio.com
Also What does your caller ID read ie when you call does it say "so and so debt services" perhaps changing the caller id with your telephone provider to something that does not look like a collection agency would be worth while. Also keep in mind when dealing with e-mail any mention of debt or financial help will often end up in spam or junk folders and never read. When leaving a message on their telephone its always a great idea to point that out to them that you will be e-mailing them and it may end iup in their spam/junk folder.
I hope that was helpful. Do you have a website?
Feel free to call for follow up if requires