A method and system for selectively applying a cellular-PBX integration service. When a radio access network (RAN) receives a call request seeking to set up a call for a cellular wireless communication device (WCD) that is operating in the coverage area of the RAN, a determination is made as to whether the WCD is currently checked-into a cellular-PBX integration service. A service control point (SCP), for instance, may make the determination. If the determination is that the WCD is currently checked-in, then the call is set up to an IP PBX server that services the WCD, and the IP PBX server may then handle the call as it would handle a call placed to any IP PBX extension. Further, differential billing can be applied, charging an enterprise billing-account for the call if the WCD is checked-in, and charging a WCD billing-account for the call if the WCD is not checked-in.