The use of Iowa’s new 6-4-1 area code becomes mandatory on Sunday. .For the last five months callers could still get through to homes and businesses in the new area code in the middle of the state if they dialed the old 5-1-5 code. That phase-in period ends Sunday and you’ll be required to dial 6-4-1 to get through. Iowa Utilities Board spokesman Rob Hillesland says if you forget and dial 5-1-5, you’ll get a recorded reminder message. That message will expire in three to four months and you’ll then get an error message.The 6-4-1 area code was split off to meet a growing demand for new phone numbers.