How do I reach an elderly woman who won't answer my calls?

[Mitchell Snyder]

I got a lead on a house owned by an elderly woman who left a voicemail and won't pick up the phone when I try to call her back. Her son/grandson is even the voice on her voicemail. I have seen the outside of the house and it looks like a tear down and new construction project, so I already know my maximum offer. Do I mail her a formal letter of my offer, or do I just keep calling her until I reach her? I have her mailing address, because PropStream has it in the database.