But there is the Vodafone Passport service, which (providing you're on an allied network) charges you 75p for call initiation, and then uses normal within-the-UK charging for the actual minutes. If you have free minutes, it'll even use those first. And I do have plenty of free minutes. So, as a hypothetical example, spending 37 minutes 18 seconds speaking to my Mum (an Orange mobile user) should cost me 75p.
It appears that Passport wasn't active on my account.
Happily, I had called the morning before we went to check that Passport was enabled (it was a pretty new account, I'd requested Passport when it was set up, but it never hurts to check these things), and had been assured that it was. That call is, of course, on their records, with the result that my bill will be recalculated.