Hi, welcome back to the forums!
I would use your best judgement when contacting collection agencies that you have sent letters to. Depending on which 'validation' letter you used, contacting them by phone might be a bad idea. Especially since in most of the letters it states that you only wish to correspond by USPS mail.
Do you still have that letter? Is there anything in it that is person? If its okay with you, post the letter. Depending on the tone could determine your next move.
If it's been 5 months, since you last sent the 'validation' letter, I would send one more follow-up letter. Your original request stated they had 30 days, or to remove the item. Mention that you received their
contact us ASAP letter, and that you're only writing because you're waiting to receive proof by US mail. Add that this would be your final request, and your next step is to file a complaint to get the item removed. Send that letter certified as well.
I'd also write a letter to your credit bureaus, explaining that you wrote one letter 5 months back, and you have not received proof yet. This is where you would be disputing the debt. Attach a copy of both letters to each of the credit bureaus.
Regards-
Mike