Exercising Challenge/Response code path in pam client?

Hi, I'm seeking a simple as possible config for freeradius server (version 2.1.12+dfsg-1.2ubuntu8.1 in Ubuntu 14.04) that would allow me to exercise the Challenge/Response path in the pam client (packaged as libpam-radius-auth-1.3.17). I see PW_ACCESS_CHALLENGE in several modules, but I haven't made the leap on how to configure the server to have pam-radius-auth (currently successfully authenticating against the server) receive an Access-Challenge instead of Accept/Reject. My end goal is verification of the client side code, rather than a production server deployment. Could you please provide me with guidance? Thanks Alan, and others, for both these pieces of software. - Richard Perrin