OK so I have an Officeserv PBX, connected via SIP. This is behind a 2830vn router, or WAS until recently. With the 2830vn we've had an ongoing problem of incoming calls returning "number unrecognized". I'm almost certain this is due to the inbuilt VoIP facility trying to take the call and failing.

Alternatively could it be the vague SIP "firmware problem" being discussed elsewhere on the forum?

Disabling the inbuilt VoIP via telnet helped slightly in that it just gave silence instead of "not recognized". Also there's a list of port redirects configured that according to our phone provider should allow the PBX to work behind NAT.

A substitute 2760n works. First time!

I do not want to use this router permanently though, I want to reconfigure my 2830vn to pass on inbound SIP correctly.

You need to disable SIP on the router I think so that it doesn't intercept (reasonably, as a VoIP router, it assumes you want to use it ).
I can't remember how but I think it might be a telnet command.

I tried the telnet command, it did something as the issue went from number unrecognised to silence. It seems as if even when disabled the VoIP module "eats" the connection attempt.

Next thing I'd like to try is can I configure the VoIP facility to be located LAN-side instead of WAN-side, so it wouldn't get in the way, and as a bonus could possibly be used as an extension?