As of Feb. 21, Marin County residents have to do a little extra when making a phone call.

In December of 2013, the California Public Utilities Commission added a new 628 area code to the same geographic region as the current 415 area code of Marin County to meet increasing demand for cell phones. While the 628 area code won’t begin to be assigned to new Marin County customers until March 21, the new dialing system is effective as of Feb. 21.

The addition of the 628 area code requires that all Marin residents begin their dialing with a one and the area code, even if they are dialing a 415 number within the 415 area zone. Customers who do not include the area code will be instructed to hang up and dial again.

The addition of a new area code won’t change the phone numbers of existing customers who received their number prior to February of 2015, and all calls that are within a customer’s local calling area will remain local calls, keeping the cost the same, according to the California Public Utilities Commission. Dialing procedures for numbers such as 9-1-1 will remain unchanged.

The original 415 area code was established in 1947 and includes San Francisco, much of Marin County, and a small portion of San Mateo County. The area code addition will affect Belvedere, Corte Madera, Fairfax, Larkspur, Mill Valley, and San Francisco, among other Bay Area cities.