I know its a long time ago, but I am trying to get this working. It sounds like you had a problem with your routing table but I had the same problem the other day I could ssh to the internal IP from a remote network over openVPN but the SIP client would not connect over the openVPN link. I didnt get round to a full debug, I thought I would be lazy and ask first...

To complete: This was bug 1860. I applied the workarounds contain therein and I get a VPN connection with a SIP session running over the N900 SIP stack.

I added the libmissioncontrol-utils with this command

#apt-get install libmissioncontrol-utils

this allows the use of mc-tool commands
then added the lines

script security 2
up /etc/openvpn/nokia.up
down /etc/openvpn/nokia.down

to client.ovpn config file

then created the startup scripts nokia.up

#!/bin/sh
run-standalone.sh /usr/bin/mc-tool update sofiasip/sip/_xxxx string:local-ip-address=$4
run-standalone.sh /usr/bin/mc-tool enable sofiasip/sip/_xxxx
route add -net 192.168.2.0 netmask 255.255.255.0 gw 10.8.0.1 dev tun0

and nokia.down

#!/bin/sh
run-standalone.sh /usr/bin/mc-tool disable sofiasip/sip/_xxxx

then these two scripts must be made executable

#chmod 744 nokia*

thanks to KWEK and others who have been through this before. By the way for completeness the command

$mc-tool list

will give you the sofiasip files that you created when you made the SIP client on the phone, these must be used where I have put _xxxx in the above scripts...