How to transfer calls from iPhone to iMac

This link will tell you --> Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

Edit: Didn't notice till I hit reply that ckuan already posted the link, but now you know two of us know the answer.

Make or answer a call on your iPhone, just like normal.

Mid-call, tap the Audio button to bring up more options.

Tap on the iPad or Mac that you want to transfer the call to.

Read here => Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support