How can I silence all incoming calls from a single contact without having to block them?

I have a very dear elderly relative who gets lonely and will frequently call me several times throughout the day while I am working and unavailable to talk.  How can I silence this one contact without blocking them?  I do want them to be able to leave me voicemail messages, so that I may be aware of their concerns, and then address those concerns with them at a more appropriate time for me to talk with them.