iPhone and Exchange ActiveSync: not yet solved - router update

Following all the links in the chain...

iPhone --  Router NAT -- (Routing and Remote Access Firewall) -- ActiveSync -- Exchange

Error 3033 for ActiveSync suggested checking the router HTTP(S) timeout.  The router is a Draytek Vigor 2900.  There appeared to be no way to alter the HTTP(S) timeout.  I have since been told that there is a command I can run...

portmaptime -s 900

So I telnetted into the router... type ? for help... the command is not listed... I had a look at the commands whilst I was there... nothing... firmware is 3.1.2 ... the service tech tells me that the portmaptime command was introduced in a later firmware.

Portmaptime -s will alter the TCP SYN Timeout value.  From the Draytek forums it appears that this command should work on the 2800 Series (2820) and other Vigor routers too.

Microsoft recommend a value of 900 seconds.  But there's still no guarantee that upgrading the router firmware and altering the timeout is going to resolve the iPhone<->Exchange problem altogether.  Have to wait and see.