If you have already have set up your primary email access on your iPhone, trying going into your iPhone calendar, tap onto Calendars (bottom middle section of the screen) and see if the delegated calendar is there.
You can do this as follows if you are using Exchange Online in Office 365/Microsoft 365 and both users are in 365
1.) Install the Outlook App for IOS. You can add this via the Apple App Store 2.) In Calendars, go to menu (3 lines top left) and Add Calendar 3.) Select Add Shared Calendars 4.) Enter the username/email address of the person you have delegate over in your organisation
Their calendar is now visible on your IPhone alongside your calendar. You should have delegate permissions if it’s been set previously.