SOL Expiry Letter

I received the letter from PRA on 8/14. They are trying to collect the debt from 15 years ago. Same time I'm receving many calls from now which I never answer them. I'm planning to send the SOL Expirty letter. Now I'm in Washington but the state that I opened the account was back in California. Lastly, even on the letter I got from them said The law limits how long you can be sued on a debt and how long a debt can appear on your credit report. Due to the age of this debt, we will not sue you for it or report payment or non-payment of it to a credit buareu. Depending on the laws of your state, certain actions, such as making a payment or promising to pay the debt, may restart the time period for the filling of a lawsuit against you; but even if that were the case, we still will not sue you on this debt. It sounded very nice which I don't believe them at all. Same time three credit buareus are no longer showing on this debt. At this point do you still recommend that I should send out SOL Expirty letter ? Which state do I have to mention in the letter ? WA or CA ? I'm very sure that they are gonna start calling me everyday. I already ignored few calls previously and now they called me one tim this Saturday and Sunday as well. Please let me know your advice ASAP ! Thank you so much for your valuable time.