An on-call locksmith is one who is available outside normal working hours, at night, at weekends or on bank holidays, to deal with call-outs that cannot wait until the next day.
In many firms the on-call cover is organised in shifts, so there is always someone you can reach even when the workshop is closed. What you should ask for is the same as during the day: to be told before they start what they are going to do and what you are going to pay.
In our case we are available 24 hours a day, every day, with a fixed quote before we start. If it is the middle of the night and you are locked out, message us on WhatsApp with a photo of the door and the lock. More under emergencies.