i assume this is typical server-to-server [ rather than dialup-alike ] setup.
on the server side put something as follows:
# first local address assigned to the vpn tunnel, then remote
# [ this is not SIPx/CIPx - it's just private addressing for the tunnel ]
ifconfig 10.255.255.10 10.255.255.9
# here you install a new route on the server whenever vpn is established
# you want to put CIP1/32 routed via vpn ip assigned to the remote end of vpn
route 10.13.0.0 255.255.0.0 10.255.255.9
on the client side:
# put your SIP1 here and some port on which server is configured
remote 213.xxx.xxx.xxx somePort
# again - first is local, second - remote address of vpn tunnel
ifconfig 10.255.255.9 10.255.255.10
# you want to put below SIP1/32 routed via vpn ip from far end of the tunnel
route 10.15.0.0 255.255.0.0 10.255.255.10
you will need to add to those few lines specific to your encryption method. try for starters pre-shared key as described here. it'll be more secure if you use proper tls for production setup.
since you'll inject SIP1/32 [ SIP1 255.255.255.255 in notation used in openvpn ] - your secure route - as soon as vpn is established - will always be chosen for communication to SIP1 [ and vice versa ].
but.. as soon as vpn goes doen - possibly sensitive traffic will flow via untrusted network. which.. you do not want to happen. if you do have to use public SIP1 / CIP1 - make sure firewalls [even the local ones] allow only communication over the tunX devices created by openvpn and do not let any direct traffic on public internet.