Porting your telephone number requires it to be on the same switch. If the switching system your phone number is on, is actually for a different area than you are actually living in, then Cox won't be able to port it. For example,
If you are on OOMA or VONAGE, and you are living in Gloucester Va, and move to Richmond, Vonage and ooma can keep your number the same, BUT, Wire phone companies, will not be able to, as the SWITCH for each area is different.
https://www.fcc.gov/consumers/guides/porting-keeping-your-phone-number-when-you-change-providers