Is it permissible for a Muslim man to marry a Christian woman if her guardian (wali) is absent, and is it valid for her guardian to deputize a Christian man over the phone to be her guardian (wali) for the marriage contract?

It is permissible for the guardian of a Kitabiyyah (a woman from the People of the Book) to appoint another person as his agent for her marriage. The agent then stands in his place and marries the woman. The Kitabi guardian must meet the same conditions as a Muslim guardian, and agency by phone is permissible.