In the UK, we have quite a few 'on call' firefighters (about a third of the total) who are fully trained and are paid but are not full time. They typically work another job but are available during certain hours if required. I'm not sure if similar systems exist elsewhere.
Looks like 80% of firefighters (or rescue personnel according to modern naming) in Finland are members of volunteer fire brigades or so-called contract fire brigades and they respond to nearly half of the total emergencies but over 90% of emergencies outside densely populated areas.
A huge portion of the US is covered by purely volunteer fire departments. Communities pay for equipment and training and that is it. Response times are slow but it is really the only viable option in rural areas.