Constant egaged tones with NPower's GN's, especially 01668 numbers, can be overcome by using the Main Switchboard number, 01793 877777, and asking for the service you need.
I tried for ages to use 01668 681112, but the number appeared to be constantly engaged!
When I tried the 08457 number given in my gas bill, I was connected straight away!!, which tends to suggest that the number is artificially engaged.
So I dialled the main switchboard, asked for Gas Customer Services, and was connected without too much fuss, apart from the usual 'Press 1 for this service' 'Press 2 for that service', but as my phone service provider gives me so-called free geographical phone calls, I wasn't worried.
Hope this is useful to anyone else on free call tariffs.

I have just signed with npower, and I was sent my document confirming this. The contact number is 0845 601 1102. I called the sales number (0800) to complain about this and threatening to cancel the contract if they did not provide a geographical number. After a brief "consultation" the girl who took the call gave me the customer sevices number (also 0800) and told me that I could use that for any calls I need to make to them. I have not yet tested this out, but when I have done so I will post it.
She would not be drawn into a discussion about "local/premium rate" stuff, just kept reminding me that I had the freephone number, so I suspect she was told what to do, and what not to do. (Note that it is possible that this may be localised ie specific to my geographical location, although I can't be sure). For information the number is 0800 553 322.

Don't know where you are but Google brings up the below for 0800 55 33 22:

Northern Electric
Eighth Avenue
Team Valley Trading Estate
Gateshead
Tyne and Wear
Tel: 0800 553322

Thanks Heinz - I am in Cheshire, so perhaps not!