When running a WCF service under IIS7, you need to attach your debugger to the w3wp.exe process, from an instance with administrative privileges. If you left the debugger running for a long time before calling the service from your phone, most likely the process was shutdown, and your debugger was no longer attached.
If that's the case, you can always modify the recycling condition settings of your application pools, so the process will remain alive for a longer time.