Hello @essell,
I do not want to answer for @michaelkourlas here, but porting to iOS has its challenges.
- To develop for iOS you need an iPhone and an Apple computer of some sort with which you can code.
- On top of that you must pay $USD 100 every year to maintain the right to publish the application on Apple’s App Store.
So for a “free” project, there might be a lot to invest in, and that, every year. Open source does not necessarily mean free, but you know what I mean. 
If I am right, the actual project (GitHub - michaelkourlas/voipms-sms-client: Popular Android messaging app for VoIP.ms, a Canadian VoIP provider) is made using Kotlin. Part of it could be reused, but much work would be needed for the interface and iOS specific integration. Other solutions would be a complete rewrite using a multi-platform framework. In both cases, it would need a lot of time, and investment. And one has to have skills on these. Nothing is impossible, but time, time, time.
The Android platform is more accessible because many there is only a small one-time fee to have a developer account and access to the Play Store.
So while not impossible, it would need work. This is something @William could think about and offer the same application for iOS. Voip.ms is a business and could offer such a tool to its customers as part of the environment. This would be a plus value for many, another reason to choose voip.ms. They could outsource such a development not to burden their own team and keep focused on their core business. It could be done in a matter of weeks, especially if starting from another open-source project like the one already existing. They could even outsource the development of a fork for iOS of the existing project.
Kind regards!
Alexandre