All I want is a UK (and maybe also a USA) dial-in number which
forwards A-law (or ulaw in USA) audio packets from the PSTN to my
asterisk server without changing the audio in any way. This means no
echo cancellation.

Can anyone offer such a service?

I could do it myself with a group of ISDN2 lines or preferably an
ISDN30 connection, but this is rather expensive.

In any case, why do VOIP service providers insist on echo cancelling
their connections when this job is best done at the end points?

John