Whilst working in a potentially confined space that has a number of lines it is fairly easy to knock a wire and not notice it has come loose. There are some engineers on this forum that I am sure could give some insight in to how this can happen.
Happened to us twice. First time at our pole. Second time further back with some major works.
The second time, the broadband was fine but our phone was crossed with the local Chemist. As we were recieving hundreds of calls I changed the message on our answerphone and set it to deal with it all. Plusnet organised a call out which took serveral days and for that time we just used our mobiles. When the engineer did turn up, he said that the problem had already been fixed as the Chemist had a business service (and as they knew who the line was crossed with, had probably told their engineer). He tested everything and said it was all fine. Couple of weeks later I got a bill from plusnet for calling out the engineer unnecessarily. I rang them and explained, but had to escalate the call as the script did not cover my situation. Happily the charge was cancelled in the end.