It took some messing around and calling the local automated weather line service a few times to get this figured out (I didn't want to bother a real person by calling them), but I found a solution for you.
When on a call tap the '+' button to "add call", the "add call" menu will contain your favorites, recents, contacts, and keypad, just like the phone app normally does. From there you can tell the person on the other end the number that you needed, and then just tap the green bar to go back to the call.