This was to be the next try, after swapping the power supply with another Dot. This time the app worked and the Dot connected up to the right WiFi unit. The odd thing, however, was that the Dot was live, and the colour ring responded to calls, but there was no sound (the original problem). But I did notice, on the tablet I used to setup the thing, that the calls to the Dot were being reported faithfully (in text). The Dot is paired to a Jam Classsic speaker, and on a whim, I turned it on. Almost instantly, the Dot burst into sound (Chris Barber), to almost instantly turn off when the speaker took over. From then on, all was fine. Turning off the Jam Classic, the Dot took over. It would seem that the Dot thought it was still connected to the external speaker, although this was turned off. Turning the speaker on seems to have restored the correct order of things. Very odd and something to remember. Cheers everyone for your interest. I wish I could explain what went wrong. Which still leaves me with the problem of why the 'phone app didn't work, but I'll leave that for another day.