I had interesting time last week trying to get my phone line back.
I received a phone call which was irritatingly dead. But for once there was a caller ID number, so I called them back. At least I tried to. But I couldn't dial out because the line was blocked. Their autodialler had called me up but it hadn't released the line. Not everyone knows this, but it's the caller that 'owns' the line. If you hang up on someone who calls you and they don't hang up too, the line is still held. So no matter what I did at my end - unplugging the phone, whatever - I couldn't get my line back.
I called the caller ID number on another line. A recorded message told me it was research call from a respectable market research company I've dealt with in the past. if I wanted to confirm they were legit I could call the Market Research Society. But it gave no way to contact them. So I called the MRS, who gave me the market research organization's number.
Cue rambling conversation with receptionist there who didn't know, for instance, that you can't hang up on a number if the dialler doesn't release the line. Eventually, after several sequences of them muttering to their IT people, 35 minutes later the line was freed up. Apparently this was a result of a frantic check around their call centre, checking all the lines.
It just goes to show - autodiallers are dangerous things in the wrong hands.