Pulse SMS requires an Android phone to work. Apple does not allow developers to override the default messaging app (iMessage). Pulse SMS has an iOS app, however the iOS client is designed so you can access the messages from the Android phone, on your iOS devices.
That said, Apple has rejected the app from the App Store since it requires an Android device to work. Consumers will not be able to download or use this app.
Pulse SMS has a PWA (progressive web app), which works perfectly fine for sending and receiving messages through the web interface on your device. To set it up:
1. Open https://pulsesms.app from Safari on your phone. 2. Open the menu and select the option to "Add to homescreen". 3. Go through the process and a Pulse SMS icon will appear on your homescreen, allowing you to quickly access your messages through an app-like experience, outside of Safari.