Unexplained line item from Lebara Switzerland

These line items showed up recently on my Lebara Switzerland account page:

03-08-2011 00:04 Premium SMS received from 001111100000 UK 00:00:00 -4.90
02-07-2011 00:04 Premium SMS received from 001111100000 UK 00:00:00 -4.90

I’ve asked Lebara Switzerland about it twice, but they’ve ignored the requests so far. Any idea what to do about this?