I had the same problem and quickly figured it out. you have to dial a number first, like 1 or area code or something to bring up the dialer screen. The top right corner shows 123. If you press shift it will change to ABC. Then you type out like normal. To change it back to numbers just press shift again and you will see it change to 123 again.
it will dial out the correct numbers it needs to when you press talk.
Wow!! Thank you so much. This is a real time saver. I've been dialing for the operator since i got my phone in November I even took it into a Sprint store and they had NO IDEA that there is a way to do this.