Location where I can test iPhone satellite texting

I’m developing a backpacking app that relies on satellite texting and I need to get to a place to test it. Are there areas in the Bay Area that have zero cell coverage so I can reliably use satellite iPhone texting? If you know of any areas or even suggestions of places to try, I’d really appreciate it.

Preferably in the east bay hills or Marin county. I’m on the AT&T network if that is helpful. Thanks!