Dummy provider does not like triggering more than one call
Sending SIGUSR1 to the calls process will trigger an incoming call if the dummy provider is in use.
Sending SIGUSR1 a second time, while the first call is not yet answered/rejected leads to infinite ringing, user controls (accept button, reject button) not working and general weirdness.
** (sm.puri.Calls:2996): CRITICAL **: 09:09:46.290: calls_dummy_call_answer: assertion 'self->state == CALLS_CALL_STATE_INCOMING' failed
found during investigation into #195