SIGFOX is a narrow band system using the license exempt 868MHz band in the UK (and EU in general), it's low power and long range so is designed for deployments that can be powered off batteries or power scavenging techniques and then left alone to monitor whatever they're designed to monitor.
SIGFOX supports 140 uplink messages per day (12 bytes in size, bit packed format determined by the user) and 4 downlink messages per day (8 bytes bit packed format). Uplink is from the remote site to the SIGFOX network and downlink is from the central service, though SIGFOX to the remote service.
There are other competing technologies such as LoRaWAN and specifically The Things Network, but their service sits on top of existing ISP connections, so there may be regulatory and other issues as well as a lot of unlicensed operators out there.