How do I get my deleted Calendar back on my iPhone?

You can't do that through an iPhone.

Log into the Hotmail account using a web browser and check if the calendar exists.

Here's how you can recover items from the Recoverable Items folder:

1. In Outlook, go to your email folder list and then select Deleted Items.
2. On the Folder tab, select Recover Deleted Items.
3. Select the calendar items that you want to recover and then select Restore Selected Items.

Calendar Items Deleted - Microsoft Tech Community