I was being slightly facetious by not mentioning the qualifiers, but it did
work for me. The qualifiers are that the Betamax/Dellmont/Finarea offerings
have 'freedays': you topup £10 and you get some number of 'freedays' (exact
number various by brand, but something like 120). Some subset of cheap
destinations (eg landlines in developed countries) are then free for the
appointed period. After that period they're not free, but still cheap (eg
1p/min).
And the bit about working on my phone is also not quite the whole story...
it's a phone running Android 2.3 which supports SIP natively - you can
choose whether an outgoing call is a mobile call or an internet call. Put
in the Voipgain/etc details for internet calling, and it just works.
Assuming you have decent wifi, which I did. If you don't have a smartphone,
a laptop/tablet/etc would do just as well with the appropriate voip app.
Theo