I was interested to read this thread because I am experiencing the same problem, disappearing credit. I have a Spanish ‘pay as you go’ Vodafone mobile which I use when on business in Spain. I usually put 50 Euros in it when ever I travel from France. Recently the credit just evaporates after a short period of time with very few calls. I have also a French SFR subscription phone and find it’s cheaper to use that even though the call costs in Spain are high.
So be warned folks either Vodafone or someone else is stealing credit on these type of phones in Spain so I’ve chucked mine in a drawer never to be used again. Very irritating.