Dialling tone

[#1]

I have just noticed that I do not have what I think is the normal (single tone) dialling tone. It is now a two tone sound hi/lo,hi/lo etc. Ant one had experience of this and is it anything to worry about?

[#2]

have you got 1571 installed:?,  it could mean someone has left you a message

[#3]

If I leave the phone off the hook for a period by accident then the dial tone changes to that high/low one.
(usually when I've failed to replace the receiver correctly)
Useful for alerting me to the fact that I've done it

[#4]

It's called a Howler

[#5]

Oddly enough we had this same thing for a few days. It did not effect the calls or broadband in anyway.
I Put it down to the BT engineers who seem to be fiddling around in the Box at the top of our road every few days. I can't imagine the cost in man hours they spend on that one box alone.