Data Validation

Our company is tracking accounts by phone numbers. What I would like is the basic formula to perform data validation. Cell 2 contains Phone number. what I would like know, what would be the formula in the data validation screen to verify that the number has not been already entered.

Eg if 555-1212 was in the c2 column, I would like to check on the next input of data, that the entry did not exist in the c2 column..

Thanks in Advance