My understanding is that the "Redial" softkey can only be used to call the most recent caller if it happens to have been from an internal number in the same cluster.
I also believe that the Redial softkey will not work if the most recent call was from an external caller because it will not automatically prepend the "9" to get the outside line.
Having said that is their in fact any way to the "Redial" feature to do just that? (ie: prepend the 9 for external callers)
You can follow this document for that:
Hope that helps, if it does please rate the post.