This is a little complicated.

Bootstrap defines icon names. Joomla has different icon names. There are about 40 names which overlap, but the icons are different for some of them. 

Most of the Joomla icon names have icons in both Glyphicons and FontAwesome - it is only the names that are missing.

It should be the responsibility of whoever integrates bootstrap into their templates to add the mappings for the missing Joomla icons names into the FontAwesome or Glyphicons icons. But generally they don't.

I have provided a compatibility mapping to T3, and posted in the Joomla forums.

But I can't fix the world.