We can tell whether the number is iMessage compatible (similar to how when you type in a new number to text on an iPhone you can tell whether it is iMessage compatible or not). So if it's not iMessage, we can send that number an SMS like normal. If they are iMessage, but for some reason delivery fails, we can also send an SMS as a backup.

Happy to chat more on privacy depending on your use case and how you want to set it up!