Hello! 
I have a problem ... 
"Challenge Not yet complete... here's what's wrong: 
The 'Phone__x' external object is not correctly setup with an indirect relationship to the User standard object."
In my opinion I do everything alright ... 
I will show everything what I am doing step-by-step, and maybe somebody help me with this issue :)
1. Creating an OData 2.0 external data source with 'Mobile Devices' as the label
2. Validate and Syncing 
3. Changing APIs from phones__x to phone__x.
4. Adding phones tab.
5. Creating User Custom Field Phone UUID with full visibility
6. Editing phone__x UUID field and adding indirect lookout relationship -> user -> phone_uuid__c, visible for all
7. Adding value '0000123442' for 'Phone_UUID__c' field.

CHECKING
"Challenge Not yet complete... here's what's wrong: 
The 'Phone__x' external object is not correctly setup with an indirect relationship to the User standard object."

8. Soo I am trying to check my phones tab, I read somewhere about empty fields there... 
8a. First is empty...
8b. Last got something...
9. Now I am changing value for 'Phone_UUID__c' field again and typing default value as '44'

CHECKING AGAIN
"Challenge Not yet complete... here's what's wrong: 
The 'Phone__x' external object is not correctly setup with an indirect relationship to the User standard object."

10. In somebody solve I find value of phone external ID, so ... lets try with ID, which is not empty
10a. Adding 'NTczMWJmZTNmODZjMmMwMzAwZDM4ZDBk' as value for 'Phone_UUID__c' field.

AND ??? ...

Challenge Not yet complete... here's what's wrong: 
The 'Phone__x' external object is not correctly setup with an indirect relationship to the User standard object.

Any ideas ? :)