Thanks for all the suggestions. I've set up a few emergency contacts on my callout phone (main work number and a couple of colleagues), but I haven't tested it yet as I'm on holiday.
I can't have anythng permanently done to the switch as the phone doesn't belong to me - it is provided by my employer (otherwise I would have superglued it already!)
A case might work, but as I mentioned in an earlier post, it makes the phone bulky and it tends to pull my credit card wallet out when I pull the phone out of my pocket (and sometimes my keys).
If only there was a simple "Disable side switch mute" option on the settings.... But Apple knows best.