FaceTime confusion: two iPhones - 'Caller ID' & 'Start new calls from'?

Bit confused here.

I have two iPhones (for different countries) a Macbook Pro and an iPad. FT is enabled on all and on each of the phones I have JUST the individual phone numbers enabled to be reached at and on the MBP I have both phone numbers checked. For ease of reference let's say phone number one is 1234 and phone two 5678.

Now I would expect that if I selected phone number 1234 to 'Start new calls' from, FT would use THAT number as the caller ID. However when I do FT automatically selects the other number in FT as caller ID. e.g.

Select 1234 to 'Start Calls From' and under 'Cellular Phone Calls' it says will use 5678 as caller ID?

Select 5678 to 'Start Calls From' and under 'Cellular Phone Calls' it says will use 1234 as caller ID?

My two main concerns are that a) the call is initiated from the right device and that b) Caller ID shows as it COMING from that number so they call the right number back? What's the point of initiating a call from one phone and FT using the Caller ID of another one.

As it is I called my 1234 phone with 5678 on FT and FT both initiated the call from the right device and showed the correct caller ID on the device being called. Not as it says it will on FT. Either way, it seems a bit botched somewhere.