For some time, Apple has been serious about building more wireless components for its own devices, putting its own Bluetooth and Wi-Fi chips into the latest Apple Watches and AirPod earbuds.

Now Apple has provided the clearest evidence yet that it is working on one of the most complicated and expensive hardware ingredients in its devices: a cellular modem. In a job listing posted a week ago, which hasn’t previously been reported, the company said it is looking for a cellular modem systems architect to work in its San Diego office. And a person briefed on Apple’s plans confirmed that the company, indeed, has an an active project to build its own cellular modem chip, which connects Apple’s phones to the networks of wireless carriers. Due to the complexity of the chip, it could take Apple as long as three years to ship iPhones with them, analysts said.