It's a closed protocol but there's lots of equipment available for it and it's being rolled out in lots of countries, a device can send up to 140 messages which have a payload of up to 12 bytes per day. Note the payload can be zero bytes, which will just indicate the device is alive. The network can send a maximum of 4 messages to devices per day and the payload is up to b bytes.
In the UK the SIGFOX network was run by Arqiva, but they didn't move very quickly and they didn't understand start-ups particularly well, so now WND UK has taken up the reigns with a very aggressive roll-out plan, with a fully funded commitment to cover 94.8% of the UK by end 2018.
Even though the roll-out is aggressive, some areas may not be covered and this may not suit someone developing a SIGFOX solution, so WND is now offering small companies who are developing IoT proof-of-concept solution a FREE SIGFOX gateway and licenses for up to 100 devices to access the network. This actually works well for WND, as every gateway added increases network coverage and even if multiple gateways cover an area, they are mainly receiving messages so the network would sort that a message is a duplicate.
With LoRa networks already springing up (generally piggybacking on generic home or business connections), this gives SIGFOX the ability to do the same and may get some customers who are still awaiting solutions from the mobile networks with their LTE-M or NB-IoT solutions slowly coming on-line.
The WND contact for a SIGFOX PoC solution is Tim Harris.