Your next step is to have a little chat with the manager who called you, and this chat should not be on a speakerphone. If the manager set this incident up intentionally, it is a bad sign; and if he had no such intent, it is an equally bad sign. The former scenario points to malice, while the latter points to incompetence.
Either way, let him know that you were extremely disappointed over the way this call was handled. For future reference or references, he needs to know that when he uses a speakerphone, he should open his calls by saying so. In addition, now that he has dragged you into an assessment of the employee in question, your comments should not be left hanging. You and this other manager should meet with the employee, discuss performance and behavioral issues that led to your comments, and focus the conversation on developing a plan of correction.
At the same time, most people can tell if they are on a speakerphone. If you are ever in doubt, especially with this manager, you can ask. If you do not want your comments on a speakerphone, all you have to do is speak up.
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