According to http://www.tellows.co.uk/num/09830501760
It is a 'premium rate service: sexual entertainment'
It doesn't come up anywhere else on google, so it is unlikely that it is a common scam.
Unlikely, but just worth checking that they don't have mobile phone handsets that are on BT that could potentially be running some dodgy software?
Do they have wireless phones that someone else could be using to call porno chat?
Does anyone else have access to the house, particularly anyone who comes and does work for them at weekends?
Or call forwarding set up in some way so that calls can be made remotely?
I would also go round all the extensions in their house checking what is plugged into them, anything other than the broadband router and phones, you want to unplug obviously. Check they haven't left an old computer turned on with a modem plugged into the wall or something stupid like that.
Otherwise, there is surely no way that they can have made the calls when they were out, and they need to tell BT this and have the argument and hopefully get the money refunded.