LOL... my hometown was so small, we didn't have 7-digit dialing until I was nearly 15. We had FIVE-digit dialing; last number of the prefix, and last four numbers.
Can you imagine?
Anyway, here's my two cents about the phone number:
1. It's entirely possibly that she was putting into her phone's address book a number for a different area code, but entered 509 out of habit. I think this possibility is why it's so important for LE to recover her phone - to be able to read her address book and see if that particular number was dialed by being keyed in, or if it was entered under an address entry.
I wonder if MC's phone plan had a backup address book plan? I know people who have that through Verizon, and if their phone gets wiped for some reason, they can retrieve and download the numbers via (satellite?) some system that beams the numbers to the phone (I think? I have a smartphone, and it's not available for smartphones because they should be backed up to computer desktop.).
2. Earlier someone (sorry, forgot who) said, in regard to SB, something to the effect of it would be strange for MC to be calling him at home (if she was, in fact, calling him), when a 20-year old working man would likely have a cell phone. (Sorry if I completely butchered that; doing from memory.)
By the same token, I'd find it strange that a 20-year old working man would be living at home with his parents, as well as directing personal calls to the family landline.
Does anyone know that SB actually lives at the address of that landline?