macOS Sequoia 15
in the call window.Switch to an RTT call: Click 
to switch between calls.
Add an incoming caller to a current call: If you’re on a call and a new phone call notification appears, click Hold & Accept, wait for the incoming call to connect, then click Merge.
Transfer the call to your iPhone: When your Mac is near your iPhone, swipe up from the bottom to the middle of your iPhone screen and hold until you see the App Switcher. Tap the Phone app banner at the bottom of the screen.
Depending on how you set up Wi-Fi Calling, a green bar that says “Touch to return to call” might appear at the top of the screen on your iPhone when you unlock it. See the Apple Support article Apple Support article: If FaceTime isn’t working on your Mac