I just did a factory reset on my smartphone to see if it might fix issues with battery life. I configured a phone number as the lock screen code so I have a code that's both long and easy to remember. Are there security risks that make this a bad idea?
Your phone number is documented in a large number of places - your friends, your family, your employer, maybe even your Facebook profile and G+ profiles. Many of these people know your phone number. Some of them may even know it from memory.
The equivalent would be writing your password down in an email and sending it to your contacts list - you're not advertising the fact that it is your password but the information is out there and significantly easier to find than a password or PIN that you have memorized (which would require intimidation or torture to get it out of you, or brute force/exploiting a vulnerability to bypass).
Using your phone number as your PIN code exposes you mainly to password (PIN) guessing attack. Your phone number is not a strong knowledge factor. People you know and trust, and thus they know you could be the first ones to guess your password. There are so many users who use their phone number as a password to their social/messaging accounts which thing is a bad practice.