I totally agree, that's where the answer is. They need to really pull out ALL stops to find out who answered that phone, and what was said in the 50 second conversation.
(It does occur to me at this point, that she may have answered it herself and doesn't remember it. She may frequently get calls in the middle of the night, and she may have been in a sleep cycle where she didn't wake up enough really to have a good memory of it. Either way, THIS is where they need to be looking, IMHO, with all the force they can muster. They may have waited several days to ask her, if it was a routine "wrong number" call, she may just not remember it).