After a telephone call to Zen Support (talking to a very helpfull engineer), I now think that this is a BT line fault - the phone is now also very noisy and intermittent. (
I did try swaping the routers/filters and a spare router - all with the same results)
As I do not need two ADSL connections at the moment, I am happy to leave things as they are ('phone and router disconnected) until after the new year - and then try 17070, etc. (
The 'phone now has a divert from the bad line to the good one and all of my servers are happy just on the one working ADSL connection)
Thank you both for your comments.
Line One:- Zen - DrayTek Vigor 2600VG
Line Two:- EntaNet (Aquiss) - DrayTek Vigor 2600