Bootstrap modals don\'t work correctly on Android and iOS. The issue tracker acknowledges the problem but does not offer a working solution:
Modals in 2.0 are broke
Admittedly, I haven't tried any of the solutions listed above but I was (eventually) jumping for joy when I tried jschr's Bootstrap-modal project in Bootstrap 3 (linked to in the top answer). The js was giving me trouble so I abandoned it (maybe mine was a unique issue or it works fine for Bootstrap 2) but the CSS files on their own seem to do the trick in Android's native 2.3.4 browser.
In my case, I've resorted so far to using (sub-optimal) user-agent detection before using the overrides to allow expected behaviour in modern phones.
For example, if you wanted all Android phones ver 3.x and below only to use the full set of hacks you could add a class "oldPhoneModalNeeded" to the body after user agent detection using javascript and then modify jschr's Bootstrap-modal CSS properties to always have .oldPhoneModalNeeded as an ancestor.