I work for a telecom comp. supporting smartphones and I've come across this type of issue many times. It's harder to pinpoint since the reason changes why it happens but generally there are two main ones:
1. Browser type: Most websites will change the way they display pages depending on what browser is used to view the page. In order for the site to know it gets the browser title from your computer/device. It sounds like your website login doesn't support being used by mobile devices, so it may not have an entry for them. iPhones use the safari browser, which is on the Mac (a supported browser). So that may account for it. Also, the iPhone in general handles web pages more like a computer so it can typically view pages other devices can't. Solution: For the Blackberry try changing the "browser type" option. It lets you modify this since there are comptability issues like this for pages that don't work with mobile devices. Change it to internet explorer and it will tell you if it has to do with browser type.
Hope that helps.