My parents are incredibly patient, but after a couple of these calls, they'll just leave the phone to ring. The soundtrack of the phone constantly ringing in the house, and the guilt associated with not picking up, is unbearable.
My brother and I built a system where her calls get re-routed to a rotation of relatives to answer her calls, to spread the load. After a call with her, each person gets a 2 hour break (customizable). If no one is available to answer, or if everyone is on break, she gets a voicemail that my dad recorded that explains that we love her, that she lives there, all the usual stuff.
It's working beautifully.