VOIP used to work, now it doesn't, troubleshooting help wanted

I set up a VOIP account a while ago (like months, even a year or so)
with sipdiscount.com, it worked fine and we used it for some calls.
It's been unused for a while though and when we tried to use it
yesterday it didn't work.

I've looked at all the obvious things that I can think of:-

My account at www.sipdiscount.com still works, I can log in and I
have some credit still.

The setup in my Speedtouch 716WL router seems OK, when I enter the
sipdiscount.com details I get a green tick on the web
configuration saying they're registered OK.

I've got working phones plugged into both phone ports on the
Speedtouch.

However absolutely nothing works, no dial tones on the phone, no
ringing, just silence. When I call my sipdiscount.com number from a
landline it connects OK and I hear the ring tone on the calling phone
but there's no actual connection to the phones on the Speedtouch.

Help!!! What else should I be checking, what else might be wrong?

--
Chris Green