B1ServiceLayer - get ContactEmployees by InternalCode / ContactCode

Hello everybody, what is the easiest way to get an ContactEmployee from the B1ServiceLayer? At the moment we are doing following stuff...

1. GET https://b1servicelayer:50000/b1s/v1/ServiceCalls(100)
2. .. then checkout the "CardCode" and "ContactCode"
3. GET https://b1servicelayer:50000/b1s/v1/BusinessPartners('C20000')?$select=ContactEmployees
4. Loop json array "ContactEmployees": [..] for InternalCode = ContactCode
5. Finish

How could we do that faster without creating a view for simple GET operation to the ContactEmployees?

We tried already:

• Expand with oData v2
  GET https://sl1.rz-nag.de:50000/b1s/v1/ServiceCalls(100)?$expand=ServiceCallStatus,ContactsEmployees
• Expand with oData v4
  GET .....