The C4C object identifier webservice (CommunicationServicesManagementQueryObjectIdentifierMappingIn/Query Object Identifier Mappings) only works if you supply the correct Remote and Local ID type code. Thankfully, the documentation (linked above) provides a list of type codes for commonly used objects, however, the employee business object is missing.
To make it short, the type codes for employee are:
- Local ID type = 888 (Business Partner)
- Remote ID = 3 (Employee ID)
See C4C data type IdentifierDefiningSchemeCode for a complete list of supported IDs, however this list is often ambiguous (same description for two ID types).