Call Directory Extension - Error Enabling Extension. Failed to request data

I had the same issue and as Jimi said there were no duplicate numbers. After debugging and testing I came to know that it requires the array in ascending order. I then sorted my array of phone numbers and it started working again.

Hope this help someone who is still struggling!

Well I had a similar case with a user that was trying to enable the extension from Settings -> Phone -> Call Blocking & Identification and was getting the same message as above.

What I found out, after debugging requests to our company's catalog, was that I had one or more duplicate entries such as [12345, John Doe], [12345, Jane Doe].

That didn't cause any trouble to users that already had the extension enabled on their devices (at least I had no complaints whatsoever), but made the extension impossible to enable for users that didn't have enable the extension before.

Now, I can not guarantee this is the case for your problem, but you can start by checking for duplicate entries in Directory.