To test the hypothesis that simultaneous mobile phone-based alerting of CPR-trained volunteers (Mobile-Rescuers) with Emergency Medical Service (EMS) teams leads to better outcomes in out-of-hospital cardiac arrest (OHCA) victims than EMS alerting alone. The outcomes of 730 OHCA patients were retrospectively analysed, depending on who initiated CPR: Mobile-Rescuer-initiated-CPR (n = 94), EMS-initiated-CPR (n = 359), lay bystander-initiated-CPR (n = 277). An adjusted analysis of the intervention and their main outcomes (emergency response time, return of spontaneous circulation, hospital discharge rate, neurological outcomes) was performed (Propensity Score Method with patient matching). Recruited and trained Mobile-Rescuers (n = 740) arrived at the scene in 46% of all triggered alarms. There was a significant difference in response time between Mobile-Rescuers (4 min) and EMS teams (7 min), (p < 0.001). Compared to EMS-initiated-CPR, Mobile-Rescuer-initiated-CPR patients more frequently showed a return of spontaneous circulation, but statistical significance was narrowly missed (p = 0.056). The hospital discharge rate was significantly higher with the Mobile-Rescuer (18%) vs. EMS (7%), (p = 0.049). Good neurological outcomes (Cerebral Performance Categories Score 1 and 2) were seen in 11% of Mobile-Rescuer patients and 4% of EMS patients (p = 0.165). There were no significant differences compared with lay bystander-initiated-CPR. Simultaneous alerting of nearby CPR-trained volunteers complementary to professional EMS teams can reduce both the response time and resuscitation-free interval and might improve hospital discharge rate and neurological outcomes after OHCA.