Because of the increased industrial and research communities interest in the IP Multimedia Subsystem (IMS), the needs of formal testing for IMS applications are becoming critical. In this work we take as a case study the IMS Push over Cellular (PoC) service, an OMA standard, and propose a formal testing approach to check that its implementation respects the standard requirements. The approach considers a formal model of the service as well a set of formal invariants representing the most relevant properties expected from the implementation. The invariants are verified on the PoC formal specification and then automatically checked on the execution traces of the implementation. Experimental results are then processed to provide testing verdicts on the implementation under test. Besides, we discuss the results analysis that show false positives essentially raised by inter-working SIP services.