Your iPhone and Mac are on the same Wi-Fi network and connected to the internet. See Sign in to or out of FaceTime, and the Apple Support article Make a call with Wi-Fi Calling.
You can turn off phone calls on your Mac at any time. In the FaceTime app, choose FaceTime > Preferences, click Settings, then deselect Calls from iPhone.
If you have hearing or speech difficulties, or want to call someone who does, you can use Real-Time Text (RTT) for your phone calls, if your carrier supports it.
Note: To make or receive an RTT call, you must be using macOS Mojave 10.14.2 or later with Wi-Fi Calling through iPhone with iOS 12.1.1 or later. This feature is not available in all countries or regions or supported by all carriers. Depending on your network environment, an RTT call may be downgraded to a in the pane.
When RTT is enabled on your Mac, the RTT button Make and receive RTT phone calls in FaceTime on Mac