Calibre epub with special fonts on Iphone

[Noia]

Using calibre, I created an ebook (epub) which uses some foreign language fonts (otf). They reside in a separate directory /Fonts within the epub container and are properly referenced in the stylesheet. The xhtml files look as intended when viewing them with a browser, and the epub also displays them properly on calibre's quick view. On the Iphone (Stanza) the book looks good, too -- but the extra fonts are not displayed. Has anyone found a way to make Iphone display these fonts, or is this an issue of the stanza app? Or can these fonts be uploaded some other way?
Any help is appreciated!