It might be Code Red in the 310 dialing area this month when the state’s first area code overlay goes into full effect on L.A.’s Westside.

Beginning July 26, all calls made within the 310 territory must include the area code with the number or the calls will not go through. Starting on August 26, all new numbers assigned in the 310 territory will get the new 424 area code.

The approaching deadline has caught many business owners, professionals and residents off guard, especially those with automatic dialing machines that must be reprogrammed or replaced. These include internal business systems, speed-dialing equipment, call-forwarding devices, computers, alarm systems, cell phones and any other piece of equipment that stores a phone number that can be used for automatic dialing.

“Almost nobody I’m talking to is aware of this. People are going to wake up on July 27 and find their phones or speed dialers will not work,” said Steve Diels, who runs a call center in Redondo Beach.

Hardest hit are likely to be owners or operators of facilities with secured-entry systems that rely on visitors dialing an entry code that speed dials a number. Many of these machines must be reprogrammed, while some older ones that can only store seven digits must be replaced entirely.

This has kept many telecommunications providers, security companies, property management firms and others busy in recent weeks and months, preparing for the changeover.

The Los Angeles district office of Wichita, Kan.-based Protection One Inc. has been contacting about 300 of its customers with older alarm systems to schedule appointments to reprogram their equipment.

District manager Richard Bishop said his technicians have reprogrammed all but a handful of the alarms. “The actual reprogramming is a fairly simple task; it’s the scheduling of all those appointments that really takes the time,” he said.

Things get a little more complicated and expensive when the automatic dialing equipment cannot handle the 11 digits (1 + area code + number) that will be required as of July 26.

For an entry system dialer for a small apartment or condominium complex, it typically costs about $2,000 to $3,000 to replace the equipment. Replacing systems in larger buildings that hold dozens of numbers can easily top $10,000.

That’s the situation that Los Angeles-based Sunset Telecommunications is facing with one of its customers. “This customer has an old gated entry system that only does seven-digit dialing,” said Katie Mantis, an account manager at Sunset Telecommunications. “We’ve got a proposal out to them now to replace the entire system.”