The field telephones we used at ResCon didn't belong to Mendip, they were brought down by BCRC comms officer from the North Wales team.
Single-wire phones are your cheapest and easiest way forward. I build a batch of about 30 handsets and 10 base stations for UK rescue teams that ordered them during 2013. They paid £43 per handset and £64 per base station which represents the parts rather than labour.These were designed to eliminate all known hassles with the previous generation of caving field phones:1) stop people from turning them off in the cave (complicated procedure to turn it off once it is on)2) make them last on three AA cells for over a day's use and step up to a constant 12 volts internally regardless of battery voltage droop3) turns itself off after many hours of inactivity4) can send call tones and roger beeps5) military standard waterproof mic/speaker6) circuit is potted (not the battery of course) and a single IP68 push button to control everything.It's written up in the BCA CREJ No.82 http://bcra.org.uk/pub/cregj/index.html?j=82but you need a login to download the PDF of the article. That article in Word format and other info about it is in zip file links at:http://www.linetop.co.uk/cssdata/swt.htmAs to which teams might have spare units they might give away to a good cause: Gloucester GRG took 10 handsets and SMWCRT took 8.I may have a few spare PCBs and excess components somewhere, but I've not looked at this since 2013!
I believe Cave Link is now out of production