Call center escalation excercise, using Chain of Responsibility pattern
Review

Well done, there isn't much I would change in this implementation. I would grant you the humble badge for putting yourself as LowLevel employee :) 

The lazy and null pattern are well implemented (except a small issue that could possibly introduce null -> see minor issues). The chain of responsibility looks to be by the book. You've even used a visitor pattern between the handler and the employee. Each specific employee yields different behavior handling a phone call.

Minor issues


IPhoneCall as interface seems overkill to me. This is just a POCO.
Check arguments against null in public entry points such as public PhoneCallHandler(Employee employee).
GetPhoneCall could be written more compact: return _employee.Resolve(phoneCall) || _nextCallHandler.GetPhoneCall(phoneCall);.
RegisterNext should not allow null, as this bypasses your EndOfChainHandler.Instance pattern to avoid null handling.