Is this a house phone or a landline? If it's a land-line, that's one thing.

On a cell phone, it's surprisingly easy for people to fuck that up and think they're "calling" you but they're actually initiating an email. Or calling your old number that is synced to your contacts.

She's either starting to forget things due to age, or making a tech error-- like clicking your picture in the "Contacts" or trying to call you over Facebook or something and thinking it's "calling" you.

There is, I'm pretty sure, a Facebook messenger "phone" icon.

It will "call" you and just ring and ring wherever you are logged into Facebook (which in my case is nowhere). She could be clicking that.