I like the idea of cloud sim, hushed and the like but I have to agree that there is nothing as safe as a spare phone with a PAYG sim bought from the supermarket with cash. Three have a decent tariff on PAYG of 3p per min, 2p per sms and 1p per mb of data - put that in an old iPhone/Android and you’re golden. Leave it at work if you’re not punting that day and if you need to take it home that night after, then it can become the phone for whoever is on call at work ... just remember to put a few work related names on the contact list like help desk or on call manager.
PS I have no affiliation with Three/iPhone/Android, I just have used them and think they are ok.