You can indirectly get this information from a web page and the curl command. In the past this URL has been taken down and rate limited and put behind some sort of captcha to prevent this use, so you might need to resort to other avenues like https://checkcoverage.apple.com/ in that case.
Depending on if your serial numer is 11 or 12 characters long take the last 3 or 4 characters, respectively, and feed that to the following URL after the ?cc=XXXX part. If your serial number was 12 character and ended in DJWR, you would issue this command:
curl https://support-sp.apple.com/sp/product?cc=DJWR
To get your serial number, use the following command:
system_profiler SPHardwareDataType | awk '/Serial/ {print $4}'
Thus, you could have a complicated command to query the internet if you need a single command:
curl https://support-sp.apple.com/sp/product?cc=$(
system_profiler SPHardwareDataType \
| awk '/Serial/ {print $4}' \
| cut -c 9-
)
and then run the output of that through sed to cut to the key part
curl -s https://support-sp.apple.com/sp/product?cc=$(
system_profiler SPHardwareDataType \
| awk '/Serial/ {print $4}' \
| cut -c 9-
) | sed 's|.*<configCode>\(.*\)</configCode>.*|\1|'
There used to be a private library file with these mappings so you could consult it offline, but I noticed it was gone as of 10.8.3 (and perhaps earlier) so the above trick is the only one I know that works on the current OS without third party libraries.
Some nice third party libararies provide a look up of this:
Note that as of November 2017, Apple has forced the use of https over http for this service.
Best Answer
The Mac and iPhone need to have the same iCloud account set up.
If they do, and it's still not visible remove the account from and re-configure it on the device that's behaving unexpectedly.