Using Landlines
Landline phones have been totally eviscerated from the Millennial home. Boomers, on the other hand, are stuck to their landline and answering machines.
Sure, they have a cell phone, but they'd much rather have you call them on their landline and leave a message if they're not home. It's a little more inconvenient, but it's what they're used to, and change isn't their forte.