Could be a number of reasons. Dialler on your system, or more sinister, someone using a link via your telephone supply line.
I had a similar problem once with suspect phone calls and an increased payment demand. In my case BT did a 'thorough investigation', but came up with nothing really. At that time, there was engineers working on the telephone lines in the area, and my unfounded suspicions about that activity came to nothing. Funny really, the problem soon cleared up after my complaint.
Would suggest, the few seconds could be an 'handshake'.
Have you notified your supplier, or perhaps tried one of the suspect numbers.Best to leave your supplier to trace the number, just in case!.