Thanks Simon,
That's what we do with Gradwell already.
We have an incoming trunk from them into our co-located Asterisk box for incoming calls, and have another trunk for outgoing.
The annoying thing is that we have had no noticable problems with another VoIP trunk supplier (I can't say who -- we are one of their wholesale distributors so it would be wrong to do so).
You might think that points the finger firmly at Gradwell, but I don't think they are at fault at all. Rebooting the router solves the problem for a while, for example, and when listening to recordings of the calls made on our Asterisk server, both sides of the conversation can clearly be heard (mostly consisting of "hello? Hello? Can you hear me?/Yes I can hear you but you can't seem to hear me" sort of thing).
So that kind of points to the SIP phone to Asterisk side, except that the one-way audio can be in either direction, so both sides reach the server. So that kind of points to the Asterisk server itself, except that when using the other VoIP trunk supplier we never have any problems at all, as previously mentioned, and we are back full circle.
And on top of all that, in all of the above cases there's a potential additional "but.." or "except..." that could implicate something else as the possible culprit.
Bah! Maybe our problem is just a particularly unfortunate combination of phones, router, trunk supplier and Asterisk version which happens not to work in harmony for some reason.
At least it happens very infrequently at the moment, especially as long as I remember to do weekly router reboots.
Faris.
