TO PROVISION EXISTING PHONE(S):

Under your 3CX console

Go to "Phones" > Select a specific extension > Click on "Phone Provisioning" tab.

Under "IP Phone" section of "Phone Provisioning" tab, change the "Provisioning Method" to "3CX SBC (Remote)"  [There are currently 3 options: 1. Local Lan (in the office), 2. 3CX SBC (Remote) & 3.Direct SIP (STUN - remote) method.]

Once changes to "3CX SBC (Remote)" provisioning method under the "Phone Provisioning" tab >

The Mac Address would be shown below.

Port is shown as "5060" (this is correct)

Lastly, be sure to enter the correct "3CX Session Border Controller" --- IP Address. 

The IP Address here shall be same (static IP preferred) and local IP where that phone can be connected to. *** Note, SBC's Local IP will change pending on the SBC's network location, etc.

Once done. Scroll up to Click on "OK" to 'save' the setting.

Go back to "Phones" sidebar > Select the extension previous change to SBC > Select the square checkbox then click on "Reprovision" to ensure the settings is updated.

That's it.