I call my 95 year old grandma every week. She doesn't have a cell phone, only a landline. I would guess many if not most older people are similar. If the replacement service doesn't work with landlines then it's dead in the water as a replacement and will only function as an additional option. Going to stay that way for a while as (my assumption is) the group 70+ likely have a large population without call phones.

Not to mention rural communities without adequate cell service.