When I attempt to validate the challenge, I get this error:
Challenge Not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.ListException: List index out of bounds: 0

I went and looked at the External Data Source data and see this:
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?><feed xmlns:d="http://schemas.microsoft.com/ado/2007/08/dataservices" xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata" xml:base="http://phone-odata-demo.herokuapp.com/devices.svc/phones" xmlns="http://www.w3.org/2005/Atom"><id>http://phone-odata-demo.herokuapp.com/devices.svc/phones</id><title type="text">phones</title><updated>2016-02-27T04:59:09.327Z</updated><link href="phones" rel="self" title="phones"></link><entry><id>http://phone-odata-demo.herokuapp.com/devices.svc/phones('NTZkMGRjOGI1Nzc4YmYwMzAwMmMxYTk4')</id><category scheme="http://schemas.microsoft.com/ado/2007/08/dataservices/scheme" term="devices.phones"></category><link href="phones('NTZkMGRjOGI1Nzc4YmYwMzAwMmMxYTk4')" rel="edit" title="phones"></link><title></title><updated>2016-02-27T04:59:09.328Z</updated><author><name></name></author><content type="application/xml"><m:properties><d:UUID>NTZkMGRjOGI1Nzc4YmYwMzAwMmMxYTk4</d:UUID><d:Brand>HTC</d:Brand><d:PhoneNumber>555-255-5556</d:PhoneNumber></m:properties></content></entry></feed>

The debug log in Salesforce says that the data should have a UUID of '0000123442' but it is 'NTZkMGRjOGI1Nzc4YmYwMzAwMmMxYTk4' in the OData feed.

The actual SOQL query that gets ran for the validation is: 
SELECT ID, UUID__r.Name FROM Phone__x WHERE UUID__c = '0000123442' LIMIT 1

Without that data feed returning 0000123442, there is no way to pass the challenge.

Any thoughts on what I could be doing wrong to not see the correct UUID returned?