Call Duration not being recorded on Apple Device

Hello, we are using the Zendesk Calls to log calls made by our client account management team. This seems to work very well so far on users who have an Android Device. The prompt to log the call is raised after the call is made, and is synchronized with Zendesk.

However, we have a problem with users who have iPhones. For some reason, the call duration is not always logged in the call. 

Is there a solution to the above?