I work for a big UK Uni so access to Mac hardware is very easy.
Testing wise I have 2 MacBook Pro 2015 models both with i7 processors and 16GB ram with 500GB SSD. Both are now running High Sierra, both have the iMessages contact issue where you cannot send a message to a contact without the country code prefix being present in the contact, and if you receive an iMessage from a known contact it does not show the contact name, just the number.
I also have a slightly older MacBook Air, also now running High Sierra, which works perfectly. I can send and receive iMessages from my contacts and contact names display as they should.
Hardware shouldn't be an issue in this. I also had the problem in Sierra so its not a High Sierra thing either.
You can assume I have done a lot of testing, I know what I am talking about and I'm pretty much the person who sets the Apple strategy at the Uni and manages the platform so believe me when I say this is a tough nut to crack.
p.s. I have spoken to Apple within the last few months, on the phone, and they are stumped.
M.
[doublepost=1507195604][/doublepost]Here are a few things you could try to see if you have better luck than me:
ICLOUD
CONTACTS
	
		
			
		
		
	
				
			Testing wise I have 2 MacBook Pro 2015 models both with i7 processors and 16GB ram with 500GB SSD. Both are now running High Sierra, both have the iMessages contact issue where you cannot send a message to a contact without the country code prefix being present in the contact, and if you receive an iMessage from a known contact it does not show the contact name, just the number.
I also have a slightly older MacBook Air, also now running High Sierra, which works perfectly. I can send and receive iMessages from my contacts and contact names display as they should.
Hardware shouldn't be an issue in this. I also had the problem in Sierra so its not a High Sierra thing either.
You can assume I have done a lot of testing, I know what I am talking about and I'm pretty much the person who sets the Apple strategy at the Uni and manages the platform so believe me when I say this is a tough nut to crack.
p.s. I have spoken to Apple within the last few months, on the phone, and they are stumped.
M.
[doublepost=1507195604][/doublepost]Here are a few things you could try to see if you have better luck than me:
- go to https://www.icloud.com
 - login
 - go to contacts
 - click on the settings wheel bottom left
 - choose preferences
 - make sure format/address layout is set to United Kingdom, or whatever your country happens to be
 - give it some time to sync the settings across your devices, say 1 hour
 - try iMessage on your Mac again and see if it works, didn't for me, but I can see why this might work
 
ICLOUD
- log out of iCloud on all your apple devices, phones, iPad, Macs etc all of them
 - give it some time to sync, again, say 10-20 mins
 - log in to your iCloud on all your devices again
 - try iMessages, see if it works.
 
CONTACTS
- make sure your contacts are the same on all your devices
 - all my contacts are on my iCloud account
 - I only have my iCloud contacts enabled on all my devices, this is the ONLY contact list I use across all my Apple devices
 
			
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