As you may know, on IOS you cannot play a audio from a notification for longer than around 30 seconds (when your phone is off or if your not on the app in which the Notification is being played from) but if you go to the app in which the audio is being played in you will be able to play the audio from the notification until it ends, so please add a longer athan when a user is in the app so that it’ll be easier to follow the sunnah of reciting after the Mu'adhin.
—> more details on this problem: https://support.quanticapps.com/hc/en-us/articles/115003649689-The-Athan-sound-isn-t-complete
—> https://support.muslimpro.com/hc/en-us/articles/200588785-The-app-is-not-playing-the-full-Adhan

jazakallah khair