It's pretty common for traditional organised volunteer services to have "on call" aspects. Think of like volunteer firefighters, The Samaritans, St John Ambula

It's pretty common for traditional organised volunteer services to have "on call" aspects. Think of like volunteer firefighters, The Samaritans, St John Ambulance, the UK lifeguards and lifeboats (RNLI). Such organisations do usually have full time paid staff too, but the bulk of the front line work is part time volunteer.