Still no non-phone solution, SMS
Neither Skype nor anything else will handle SMS properly (send and receive), on the same number as voice calls. And you need SMS as all other messaging systems work in walled gardens, so mum's use SMS. Amazing that the mess has lasted this long.
Went through this recently, ended up having to use multiple mobile phones, which appear to be the only thing that can have a mobile like phone number. Here in Oz at least.