Use your iPhone *carrier* account to make and receive calls from this Mac when your iPhone is not nearby

I think I figured out how to fix the issue, but it's a workaround. The issue seems to be a bug.

1. On the iPhone, under Settings > Phone, enable "Allow Calls on Other Devices. Then, down below, under Allow Calls On, enabled the Mac on which you want the calls to work.

2. On the Mac, in FaceTime settings, enable the check box next to "Calls from iPhone".

3. Theoretically, at this point, the incoming call should ring on the Mac and you should be able to answer the call and be able to move it to the iPhone by pressing the green band at the top of the iPhone screen. This is method #1 described above. EXCEPT, in Mojave, when you place a call into the iPhone number, the Mac doesn't ring at this point. THEREFORE, YOU MUST REBOOT YOUR MAC at this point. Once the Mac reboots, make another inbound call into your iPhone's number. You Mac should now ring.

4. On the iPhone, under Settings > Phone, tap on Wi-Fi Calling, and enable "Add Wi-Fi Calling For Other Devices".

5. In FaceTime Preferences on the Mac, you should see a new button show up that says, "Upgrade to Wi-Fi Calling". Click on this new button. After a few seconds, you will see another button show up that says "Update Emergency Address". If you have already done that in the past, you don't have to click this button. Normally, at this point, you would be able to make an inbound call into your iPhone number, and both your computer and your Mac should ring. If you answer the call on the Mac, you CANNOT move the call to the iPhone because this is method #2 (described in the first post). This method works even if your iPhone is not on the same Wi-Fi (could be miles away) or your iPhone could even be powered down. EXCEPT, in Mojave, if you make an inbound cal into your iPhone number at this point, the Mac will either not ring at all, or it will ring, but when you answer it on the Mac, you will have a green band at the top of the iPhone's screen, tapping on which will move the call from the Mac to the iPhone. Therefore, instead of method #2 (described in the first post), method #1 is still in force. THEREFORE, YOU MUST REBOOT YOUR MAC at this point AGAIN. Once the Mac reboots, make another inbound call into your iPhone's number. Your Mac and your iPhone will now ring, but if you answer the call on the Mac, the iPhone will be completely unaware of the call being answered on the Mac; therefore, you cannot move this call from the Mac to the iPhone, which is the expected behavior.

Therefore, unlike in previous versions of macOS, which didn't require a reboot for method #1 or #2 to be enabled, in Mojave (at least in 10.14.0 and 10.14.1), a reboot is required. In order to achieve method #2, TWO reboots are required as described above.

ADDITIONALLY, once the "Upgrade to Wi-Fi Calling" button is pressed (as described above), the screen shows ugly overlapping fonts due to poor formatting. This is a purely cosmetic issue, but it looks horrible, and Apple didn't fix it in 10.4.1, even though this cosmetic issue existed in 10.14.0.

I call Apple about this, and, as most of the time, they have no clue, and their solution is to reinstall the OS, which is ridiculous. I have this issue on three Macs, and it manifested itself ONLY after each Mac was upgraded from High Sierra to Mojave.

Well, even though the Mac is ringing reliably now, after a few test calls worked fine, now when I answer the call, the calling party gets fast busy. So, it's not working. I rebooted the Mac again. After the Mac rebooted, the calls continue to ring, but answering them results in fast busy on the calling phone.

Here's the good part, the overlapping font in the FaceTime Settings window corrects itself after the third reboot, but the calls - even though they ring - cannot be answered. So, the "Wi-Fi Calling on Other Devices" feature doesn't seem to be working properly in Mojave 10.14.1.

Disabling "Add Wi-Fi Calling for Other Devices" on the iPhone, reverts the Mac back to method #1, which requires that the iPhone be on the same Wi-Fi as the Mac in order for the Mac to be able to make and receive calls. Method #1 seems to be working fine in Mojave 10.14.1, so I will be using this method for now, hoping that someone at Apple will discover the problem and report it. Because when I called Apple support tonight, they couldn't care less about documenting or reporting this issue. I am actually very disappointed about what happened to Apple support in the past few years. They are pretty clueless about macOS anymore - both the junior advisers and the senior advisers. All they can do is to read the script and make you follow it. If something is not documented already, they are at a loss and start coming up with all sorts of silly explanations. They may be able to fool 95% of users who have no clue, for technical people can see through their shenanigans immediately. Their arrogance also knows no limits. It's very sad at this point. If you can't figure stuff out yourself, and you have a complicated issue, you are out of luck.