OK hre is the scoop right here on the Calibre forum:
https://www.mobileread.com/forums/showthread.php?t=61587
This willl tell you all you need to embed your fonts in each epub you write or modify. NOTE: Calibre itself need not be the epub creater.
Please don't hesitate to email me for help at
clist@3ryd.com
I hope this will convince the nay-sayers. I had the same problems from others when I tried it so you are not alone.
Paul