settlements almost there

Just finished settling with two companies successfully, but the last one is National Credit Adjusters and they won't even answer their phone. Put on hold with promises that I am 6th, then 4th, then 1st, and then it flips back to 6 again. This continued for an hour. Very unprofessional and a sign of a company that is at minimum incompetently managed. After reading about them it seems it might be risky to send them anything - their rating on budd hibbs site isn't good. Illegal practices. But I have a one time opportunity via a relative next week to settle all my debts if done at 20-30%. So far been able to do that.

If I decide I'm throwing away money to settle with them (will still be calling and harassing me), then will a request for them to validate the debt in a certified letter work. After a certain number of days they will have to take it off my credit report?

Thanks