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Even if the CA bought the debt, they are not the OC.
I would write up a nice letter, describing in detail and quoting what portions of the FDCPA they are in violation of and they have failed to validate even after several requests so you are left with no choice but to report them to the FTC, your states AG and the AG of their state.
cc a copy to each of them.
If that doesn't get their attention send them an ITS. But only do that if you mean it.