Let's start with the iPhone. My biggest issue here is that to maintain a presence on the Apple App Store, I need to fork over $100 per year for my membership. I paid a ONE TIME fee of $25 to start my Google Play Store account. I don't really sell a boatload of these apps, since it plays to a limited market, and so to offset that I'd need to charge a LOT more for the iPhone version just to break even.
And then there's the effort would take, as the Android version was built in Java, which can't be easily ported to the iPhone. If I was going to start this project from the ground up, I'd rewrite it using FLUTTER. I've already worked on a project in Flutter and it can effortlessly create copies for both platforms. Even then, it would represent hundreds of hours of work, and for the small number of copies I sell, it probably wouldn't be worth it.
I'm not at all familiar with writing an imbedded web app, though my feeling in that Flutter would be the way to go there as well. In any event, the same issues arise, especially making back ANY money at all in a web-based effort, as I don't really see how I could monetize that all.
I started the project simply as a convenience for myself, as I'm big fan of VoIP.ms. I use Android primarily, and given the inexpensive maintenance costs of an Android project (I.E. the $25 one-time fee vs $100 per year for iOS), doing it that way was a no-brainer at the beginning.
So to conclude, I don't really have an incentive to re-write this app, either in Flutter to cover both platforms simultaneously, or writing a native iOS version from the ground up.