I think a lot will depend upon the, excuse the expression, integrity of the debt collector.
When calling, I would remind them of their agreement. If you get a bunch of double talk, I would simply advise them that you are going to take two steps...
1. File a formal complaint with the FTC, advising of their breach of agreement, and
2. File a direct dispute with them for inaccurate reporting, requiring them to put in writing confirmation that they made no such agreement, and refuse to comply.
The FTC complaint will at least require an answer, so they may want to avoid.
As for the direct dispute, it will require them to conduct a reasonable internal investigation of their records and verify in writing their position. It may simply result in verification, but perhaps someone in management will review the acts of their underling. If management does not sanction their action, and they most likely have phone recordings upon which to review the "agreement," they may come forth with some integrity.
Additionally, their phone records would be discoverable in any subsequent legal action, so it would be perilous if they had knowingly provided verification that is contrary to their own records.
Those two steps are fully and properly supported by the FCRA, and are not idle threats.