So the issue is apparently the legitimacy of the debt, if it was not included as discharged under the BK.
Was your letter to them identified as a request for debt validation under FDCPA 809(a), a dispute under FCRA 623(a)(8), or simply an informal communication?
If not, you can begin the process of requiring they conduct a reasonable investigation, and verify back to you that they consider the debt legitimate.
While that alone will not require them to provide documentation or "prove" the legitimacy of the debt, it can either requrie them to cease collection activity, if sent in the form of a DV, or require formal verification within 30 days, if sent as a direct dispute.
If they formally verify, you then have their statement that they have conducted an investigation and have basis to verify the accuracy.
If you still disagree with the debt itself, you can initiate action to get the matter before a judge, with possible civil penalties if they can be shown to have verified without conducting a reasonable investigation or that the results of their investigation would not support their verification.
If the debt is legit, it apparently remains unsatisfied, and they would be justified in their collection efforts.
Do you contest the debt?