Enabling it will allow the iPhone to send text messages using carrier-supported SMS/MMS messaging. Go to Settings > Messages and make sure iMessage, "Send as SMS", or "MMS Messages" are turned on (depending on which method you're trying to use). If your iPhone is not sending SMS messages, we recommend that you check your network settings. If your iPhone isn't sending messages, first make sure your phone has this service, as the problem might be with the Wi-Fi or cellular network, and not with the device itself.

Check also - https://emailspedia.com/iphone-is-no...ts-to-android/