The cost of sending text messages (SMS) with Twillio is $0.0075 per message (in the US). If we have a 1,000 users set a number once a day on average the cost to us will be $225 USD a month. That’s not negligible. There are some discounts with bulk purchases but it’s still a sizeable cost.

Should this be a premium feature users could purchase? It feels like an important feature that could extend reach of the solution to a segment of popoulation that has phones that can’t install the app.

Also, I don’t want to charge user per notification because that could discourage use. Or am I naive here?

We could have a “donate towards SMS notifications” prominently visible button.