Just to add that all the networks require you to use the phone within a certain period. I would also echo the advice about charging. Assuming its a modern lithium based battery then it needs charging every two months or so or it will lose capacity. If its an old style Nicad/Nimh then it will need even more frequent charging to prevent damage. In short, mobiles are not suitable for very occasional use in a car without regular maintenance. As for contacting O2 they would have been closed at that time of night.
but I made a point of buying a car charger lead to keep the phone charged up
also I did not phone O2 late at night - it was at a "normal" time of day
I'm OK to a point about being cut off through under use - but I've given O2 £20 or so in prepaid charges so that I can have a phone when I need it - it's not unreasonable at least to be able to contact O2 when I find it doesn't work
My policy nowadays is that if I can't contact a company then I don't use them - so goodbye O2 - it's been great
In one of those bizarre coincidences I was reading an article today about this. Following a change at Vodafone, O2 are now the network with the longest period before they will completely cancel the account losing any credit and the number. Vodafone will now not only disconnect after 6 months of no use but will close the account.