Because nearly everyone has a cellphone, a number of start-ups, lock companies and carmakers are betting on broad acceptance of the technology.

Schlage, a major lock maker, markets a system that lets homeowners use their mobile phones to unlock their doors from miles away, and manage their home heating and air-conditioning, lights and security cameras. Customers buy locks that are controlled by wireless radio signals sent from an Internet-connected box in their home.

Recently, Dwight Gibson, vice president for connected home solutions at Ingersoll Rand, Schlage’s parent, said that he used the system to let a friend into his house while he was sitting at his desk at work. “She thought it was magic,” he said.

Daimler-Benz now has it on its Mercedes. Zipcar, the car sharing service, has a mobile phone app that allows customers to unlock their car doors by pressing a button on their phone screen that looks like a lock. They have used it 250,000 times since it was introduced two years ago.

In October, General Motors introduced an app that lets owners of most 2011 G.M. models lock and unlock the doors and start the engine remotely. It allows car owners to warm up the engine on a frigid day or fire up the air-conditioning on a hot one from the comfort of their office cubicle, said Timothy Nixon, who oversees “infotainment” products for the automaker. “In the winter, when my wife and I went to dinner and the check came, I pulled out my phone and started the car,” he said. “By the time we got to it, it was toasty and warm.”