That has happened a couple of times on my 8700g. It sounds like the BB was working on higher priority function and got hung up on making dialing the higher priority function. About the only thing you could try is to make sure you have the most current version OS.
I just always use the phone key first then start he number. Works well for me since I use the address book alot and if you hold it a few secs from the home screen it will go directly to the address book.