I think such activity should be illegal as it adds up to harassment.
Stupidity ought to be illegal, but it unfortunately isn't. *shrug*

Did any of these agencies report your yet-to-be verified debt to the credit bureaus while pursuing collection efforts?
The first one did, which is how I found out about it. When I disputed it, per credit bureau procedure, the calls started. It was taken off, though, because the collection agency couldn't verify it. And so the cycle continues. The last gap between agencies was 5 years, I think they might finally be getting bored.

Frankly, that's the only thing I am worried about, and if they can legally do that, then I might just pay them to shut up - only 100 bucks.
If you don't dispute it in writing - and, let me point out, your letter is already two weeks old, time's a wastin' - they will report it to the Big Three.

Truth be told, they can report it to them anyway. They don't have to wait for the 30 days to elapse.

But the truth is, I would be then paying for something that I am 99 percent sure that I don't owe and that really ticks me off.
A note here as well - if you pay it, they can report it as a paid collection account. If it isn't yours, dispute, dispute, dispute. In writing.