No, you don't necessarily need a paid subscription. However, you do need a subscription that includes Exchange Online, if you are going to be querying any of the mailbox-related endpoints such as /calendar or /messages. You can sign up for a free Office 365 trial here: https://go.microsoft.com/fwlink/?linkid=403802&clcid=0x409&culture=en-us&country=us
Or better yet, get a free DEV subscription that you can reuse: https://developer.microsoft.com/en-us/microsoft-365/dev-program