Radius on non-domain machines

Hi There,

I have configured a cisco access point using RADIUS as authentication from a win2k8 R2 DC. Connecting from a domain-joined machine with a user already logged in works fine, but the issue we have is from non-domain machines (including iphones & blackberries) is it won't prompt for domain credentials. where/how can I configure this on NPS?

Thanks for your help in advance

Ben