What happens to my phone if there's a power cut?

An old-style landline could sometimes still work in a power cut because power would travel through the phone line itself. A digital phone line needs your router (and ONT where applicable) to have power, so it won't work in a power cut unless you have a battery back-up pack. Usually having a mobile phone is sufficient back up as an alternative way of making calls especially in emergencies but providers are required to assess vulnerable customers' needs and provide appropriate support measures where necessary. In some cases, this may be a battery back-up unit that keeps your phone working for at least an hour.