If you have tried to install the Mpesa application onto your iPhone you might run into an error during the registration process. Many of the issues are related to using WiFi instead of cellular connection but there is a unique error affecting mostly iPhone users especially if you are running on the latest IOS Version 15.  After trying many configuration options, I managed to find a solution to this issue. On the latest iOS, The issue is caused by the private relay technology which is featured on IOS that allows users with iOS 15, iPadOS 15, or macOS Monterey on their devices and an iCloud+ subscription, to connect to the Internet and browse with Safari in a more secure and private way.

This technology block the application from being able to automatically fetch user phone number from the device. To fix this issue, follow the steps below:

1. On your iPhone, Go to Settings then Tap on you profile to launch the Apple ID related configurations. 
2. Tap on iCloud then Private Relay (Beta) to Launch the Private relay configurations. 
3. Disable Private Relay by tapping on ON/OFF toggle. 
4. Exit setting and Launch you Mpesa application. If the application was running in the background you might need to close the app and launch it again.
5. The issue should be fixed and you can complete the login process to the Mpesa application.
6. If you prefer to have the Private relay running, You can go back to the setting and enable it. This will not have any impact on the functioning of the mPESA Application  after you have logged in.