Sharing Calendar across different M365 systems

As a consultant I have a number of M365 accounts as clients ask me to use their system and email address / calendar

Whilst people in any of those organisations can see my calendar entries and book calls around them, I cannot see a way where I can share the information from my other M365 calendars 

I can share the calendars with my various accounts and can see what I am doing at anytime but if someone is looking to book a call via Outlook they don't see my booked times with other calendars

Am I missing something obvious?