Entering numbers on any onscreen keyboard often requires you to press and hold a letter to access the number, or sometimes you can bring up a number pad by tapping on the "123" icon embedded within the keyboard. If you're using Swype on your Windows Phone or Android device, there is a super fast way to bring up a full-sized number pad. Start at the Swype button in the bottom left corner and draw a diagonal line up and to the right. Boom! Numbers.
Thanks for the tip, Mr. ViZZiATO.
Update : Looks like this only works on Android. For Windows Mobile, just tap and hold the symbol button to get the number pad (thanks Ash).