Collection Agency on my tail HELP!!

It has been quite some time so I need a refresher from you pros out there:

I got a letter from a collection agency regarding an AT&T cell phone the letter basically says:
This account has been placed with our agency for collection. It is in your best intrest to resolve this account. You can call our office at 1800 BS!

Unless you notify this office within 30 days after recieving this notice that you dispute the validity of the debt,this office will ASSume the debt is valid. This is an attempt to collect a debt. Any info obtained will be used for that purpose. You have the right to inspect your credit. This letter is from a debt collector.


WHAT IS THE LETTER I SEND THESE BOZOS? WHat is the process to shake the OC? I am out of the loop help me here. Thank You.

 

I left out some key info.
The letter also states:
If you notify this office in writing within 30 days we will obtain verification of the debt or obtain a copy of a judgement and mail you a copy of such judegment or verification. So the question is do I send them a CA Validaton letter? Or a cease and desist?

 

Re: Re: Collection Agency on my tail HELP!!

AFNI THE DEBT IS THE About a year and a half old it is about 500 bucks