Ethical considerations are not the only problem for the lawyer. Depending on the state in question, the surreptitious recording of the conversation may violate the criminal code. In Wisconsin, Sec. 968.31, Stats., allows the recording of such phone calls as long as one party consents and the recording is not for purposes of furthering a criminal or tortious act.
The former Board of Professional Responsibility for the State of Wisconsin issued a formal ethics opinion on this exact question about 15 years ago. That opinion identified as E-94-5 concluded that there was no outright prohibition of such a recording but the lawyer had to be careful to consider a variety of ramifications when making such a recording. Importantly, the opinion made clear that a lawyer may never record a phone call or meeting with a client without the client's knowledge and consent.
The bottom line is that a lawyer needs to review 968.31 and E-94-5 before deciding to record or directing the investigator to record a witness interview. The lawyer also needs to be aware that such a recording will likely become discoverable evidence!
Sidenote: While much of the local buzz centered around the potential judicial choices for state and federal court, there was an extensive evidentiary hearing on the Amos Mortier case in Federal Court this past week. If there is a practice lesson to learn it is to make sure that any immunity agreements should be court ordered, or as a second choice, written.