Thanks to everyone who replied.
Never really solved the mystery but I have moved on.
After failing to make calls to various UK numbers on Friday, I went to Claro's office on Monday. Was seen very quickly. The days of standing in line for an hour to be attended to seem to have passed.
The advisor quickly established that my ability to make long-distance calls had been "unchecked" in their computer system. She was unable to say why. She "fixed" it and said I would be able to ring abroad within half hour.
Too late to call (because of the time difference) so I tried again today (Tues). Still no joy, but was able to get through immediately using my wife's Orange cellphone, so panic over.
Went to Claro office again today. Same advisor now told me that the reason was that I had run out of credit. I pay around 450 pesos a month on a plan and have only made a few local calls (I checked on my register to make sure my wife had not used my phone). So that was impossible. She also told me that I had topped up my account by 100 pesos today. Nope! It is years since I last bought minutes other than my plan.
So obviously they do not have a clue. Tried to cancel my contract but there would be a cancellation charge of 1400 pesos, so I will wait until my contract expires in three months.