What do i do if I sent a letter to a creditor (certified return receipt) and I get no response

What do i do if I sent a letter to a creditor (certified return receipt) and I get no response. How long do I wait and what is my next step?
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you can only get certified delivery to bureaus
i am not sure what you asked for or paid for
certified delivery to the mail PO box should be paid for
PO boxs can only be that way. no body can sign
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I didn't make myself clear at all with my last email. I must have been half asleep. What I am wondering after I sent my letter to the creditors how long do they have to respond. If they don t respond at all what is my next step?
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30 days
dispute my credit report
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Can you except an email agreement from a credit agency ?
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no
usps mail.
certified delivery

call
then mail
mail then call