The test attached is an example of my abilites in c# and .net. However, it is not production ready, and there are a few known issues:

I have not been able to resolve the caching that EF Core has with the entities. This means the output is as expected first time round, but rerunning the solution outputs a longer phonebook

I have included some tests, but in a real world situation, I would implement more tests - eg I have not tested the sad path, there are no null checks.