macOS Phone app not working.

The symptoms do not establish that the Apple Account is the problem. 

Since FaceTime calls work but regular cellular calls do not, I would focus on iPhone Cellular Calls, which handles both incoming cellular calls appearing on the Mac and outgoing cellular calls made through the Mac.

On the iPhone, go to Settings >> Apps >> Phone >> Calls on Other Devices, turn on Allow Calls on Other Devices, and make sure your Mac is selected. 

Then go to Settings >> Apps >> FaceTime and confirm that FaceTime is enabled, your iPhone number is selected, and the same Apple Account is shown there as on the Mac. 

On the Mac, open FaceTime >> Settings >> General and confirm that the same Apple Account and your iPhone number are selected and that Calls from iPhone is enabled. (support.apple.com)

Make sure the iPhone has an active cellular plan, both devices have Wi-Fi enabled, and they are connected to the same network and are near each other. 

The Mac can use Wi-Fi or Ethernet provided it is connected to the same network as the iPhone. 

If you use a Commercial VPN, check that it does not interfere with local networking. (support.apple.com)

If your carrier supports Wi-Fi Calling, also check Settings >> Cellular >> Wi-Fi Calling on the iPhone. (support.apple.com)

Because the feature previously worked and later stopped, if these settings are already correct, reset the calling connection: turn Allow Calls on Other Devices off on the iPhone and Calls from iPhone off on the Mac, restart both devices, then turn the settings back on. 

Test both an incoming and an outgoing cellular call.

I would not assume that Lockdown Mode is responsible. Apple does not identify ordinary phone calls as being disabled by Lockdown Mode. (support.apple.com)

If the Mac continues to report that the devices must use the same Apple Account even though the FaceTime accounts are identical, that message alone does not establish that the accounts differ. 

At that point, I would avoid repeatedly signing out of iCloud and contact Apple Support, particularly because the feature previously worked and subsequently stopped after a macOS update.

Using the  Product Feedback - and make it known to Apple regarding this ongoing issue

Apple: Make and receive phone calls on Mac

Apple: Use iPhone to allow calls and text messages on Mac

Apple: Continuity features and requirements

Apple: Make a call with Wi-Fi Calling

To fix cellular calls and incoming alerts on your Mac and the ability to make outgoing calls from your Mac, reset the relay connection between your iPhone and Mac by toggling handoff settings and refreshing network sessions.

Step-by-Step Fixes:

Toggle "Calls on Other Devices" on iPhone:

• Open your iPhone and go to Settings > Apps > Phone > Calls on Other Devices.
• Turn off Allow Calls on Other Devices, wait 10 seconds, and turn it back on.
• Ensure your Mac is toggled ON in the device list below.

Refresh Phone Settings on Mac:

• Open the Phone app on your Mac
• In the top menu bar, click FaceTime > Settings > Calls.
• Uncheck Calls From iPhone, wait 10 seconds, and check it again.

Sign Out and Back Into FaceTime:

• In FaceTime Settings on your Mac, sign out of your Apple ID, restart your Mac, and sign back in.

Check Wi-Fi and Bluetooth:

• Make sure both your Mac and iPhone are connected to the same Wi-Fi network (and the same 2.4GHz or 5GHz band if possible).
• Ensure Bluetooth and Wi-Fi are enabled on both devices to maintain Continuity.

Check Mac Notification Settings:

• Go to System Settings > Notifications > Phone (or FaceTime) and ensure Allow Notifications is turned on. 

Disable Voice Control (if enabled):

• Some users on macOS Sequoia found that turning off Voice Control (System Settings > Accessibility > Voice Control) resolves call-routing glitches.

Check the macOS Firewall:

• Temporarily test turning off the built-in macOS Firewall (System Settings > Network > Firewall) to see if it is blocking call handshake data

More info from Apple: Make and receive phone calls on Mac, iPad, and Apple Vision Pro - Apple Support

Personally, I have no issues answering or making calls from my iPhone 14 Pro on iOS 27.0.1 from my Mac running macOS 27.0.1 (my other Mac running macOS 26.7.1 and my iPad Pro running iPadOS 27.0.1 can also make and receive calls).