A method and system for selectively applying a cellular-PBX integration service. When a radio access network (RAN) receives a call request for a served wireless communication device (WCD), call setup signaling passes to a call session control function (CSCF), and a determination is made as to whether the WCD is currently checked-into or checked-out of the cellular-PBX integration service. If the WCD is checked-in, the call is set up to an IPPBX server for handling. If the WCD is checked-out, the call is set up back to the RAN for handling. Similarly, the CSCF may receive call setup signaling in response to a call dialed to an IPPBX extension associated with the WCD, and the check-in/check-out status of the WCD may be used as a basis to decide whether to send the call to the RAN for handling or whether to return the call to the IPPBX server for handling.