Cavelink 'Radio' is digital SMS-style comms. Available from http://www.cavelink.com/cl3x_neu/index.php/en/
Can be connected to mobile phone network via SMS module. Should work to -700m with 50m aerials.
Swiss and expensive: about €1100 per unit.
Wookey got offer to test a kit in 2016. But no response since. Will follow-up by phone.
Becka asked if we can borrow ICCC's. Unfortunately their expo is exactly the same time, so not this year.
ChrisD to ask is Nicola2 is ready for testing yet.