While "The woman who lived next door was a doctor" can be reduced to
"The woman living next door was a doctor",
"The woman who called me was a doctor"
cannot be reduced to
"The woman calling me was a doctor".
Perhaps if the tense (or aspect?!) were past continuous it would make sense. But in the first example the tense is also simple past, yet the reduction is meaningful. Is there any reasoning behind this one could explain to their students?