Are en-route ATC callsigns standardized?
The 'Center' suffix is indeed more common in the US for the en-route call-signs under radar control. There is also New York Radio, for example, for the non-radar service over the Atlantic. Another example is the 'Director' suffix that is not common in the US, but is common elsewhere for the final approach controller.

In this case we need a referee, enter ICAO Annex 10 Volume II Communication Procedures:


  5.2.1.7.1.2 The unit or service shall be identified in
  accordance with the table below except that the name of the
  location or the unit/service may be omitted provided
  satisfactory communication has been established.


Unit/service available                Call sign suffix

area control centre                   CONTROL
approach control                      APPROACH
approach control radar arrivals       ARRIVAL
approach control radar departures     DEPARTURE
aerodrome control                     TOWER
surface movement control              GROUND
radar (in general)                    RADAR
precision approach radar              PRECISION
direction-finding station             HOMER
flight information service            INFORMATION
clearance delivery                    DELIVERY
apron control                         APRON
company dispatch                      DISPATCH
aeronautical station                  RADIO


Above are the standards, but nations are free to deviate. Below is from the UK AIP listing the differences:





And from the US AIP:

Facility                                         Call Sign

Airport UNICOM                                   Shannon UNICOM
FAA Flight Service Station                       Chicago Radio
Airport Traffic Control Tower                    Augusta Tower
Clearance Delivery Position (IFR)                Dallas Clearance Delivery
Ground Control Position in Tower                 Miami Ground
Radar or Nonradar Approach Control Position      Oklahoma City Approach
Radar Departure Control Position                 St. Louis Departure
FAA Air Route Traffic Control Center             Washington Center



  The U.S. does not use the term "area control service" to indicate controlled flight in controlled areas. The U.S. equivalent facility for an Area Control Centre (ACC) is an Air Route Traffic Control Center (ARTCC).




Related: Where to find the callsigns of the en route controllers?