"I understand what you mean, create a condition that checks this data before creating a new one, but my syntax is not correct, can you help me?"
You have a "GetClientById", but if it's really fetching by Id that's not ok, you want to fetch by phone number and date. Also, you then check the List.Empty - if it's True, there's no conflicting record, if it's False, then you need to bail out with a message.