Continue call on FaceTime in MacBook
You should be able to transfer an ongoing call over FaceTime from your iPhone to your MacBook. Let's double-check that you have things set up correctly.

Obviously, you'll need to use the same Apple ID on both devices and, based on your description, I assume you've got this all set. Now make sure you do the following:


On your iPhone, go to Settings
Swipe up and tap on FaceTime
Now check what options you have listed/ticked for where you can be reached by FaceTime. 
Make sure the Apple ID / iCloud account you've used on your MacBook for FaceTime is listed and ticked: 


If it's not listed, tap on Add an Email... to add it
If it is listed but not ticked, then tap on it to enable it

Exit Settings


This should rectify the issue. 

Let me know how you go.