Anyone conducting business with the FCC must be registered in FCC CORES (COmission REgistration System) and be issued an FCC Registration Number (FRN). This is not an option.

Upon registration, you will be assigned an FCC Registration Number (FRN). This number will be used to uniquely identify you in all transactions with the FCC.

The CORES (COmission REgistration System) is easy to use. The FCC has devoted a section of their website to the Amateur Radio Service. Helpful web pages include: New Users guide to ULS and Common filing tasks which offers instruction on setting up a user account, registering for a federal registration number, and filing electronically via the ULS.

Amateur licensees can search for their FRN in the ULS License System. Many web-based call sign servers, including ARRL's, also can provide this information.

All individuals who register in CORES will be issued an FRN. This number must be used in place of their Taxpayer Identification Number (TIN--typically an individual's Social Security Number) when filing applications with the FCC. New or upgrade license applicants must be registered in CORES and must provide the FRN on a license application filed through a Volunteer Examiner Coordinator or on a form submitted directly to the FCC.

Although both ULS and CORES will contain a licensee's FRN, updating information in one system will not update the other. For amateurs, CORES registration replaces ULS ''TIN/Call Sign'' registration, but the ULS will remain the Amateur Service licensing database, and only ULS will associate an individual with a particular call sign and FRN. Now that CORES and FRN's are mandatory, those registering in ULS will be redirected to CORES registration when filing electronically.

The FCC has discontinued issuing an Assigned Taxpayer Identification Number, ATIN's, which the FCC had been issuing to applicants ineligible to obtain a Social Security Number, such as foreign applicants and club station licensees. Applicants who had been using ATINs must now register in CORES. CORES will offer exemptions to amateur clubs and to foreign entities not holding a TIN/SSN.

FCC Registration Help and Instructions:
https://apps.fcc.gov/cores/html/Register_New_Account.htm    
https://apps.fcc.gov/cores/html/Register_New_FRN.htm    
https://www.fcc.gov/wireless/support/knowledge-base/universal-licensing-system-uls-resources/getting-fcc-registration