Configure network interfaces for sip trunk

I have a Debian 9.5 server that I'm trying to use as a PBX server with a sip trunk, this machine has two network interfaces, one pointing to LAN another one pointing to my sip provider. This is the configuration: iface LAN inet static address 192.168.1.247/24 gateway 192.168.1.254 # dns-* options are implemented by the resolvconf package, if installed dns-nameservers 192.168.1.254 allow-hotplug SIP iface SIP inet static address 172.xxx.xxx.xxx netmask 255.255.255.252 And a SIP server with IP: 172.xxx.xxx.xxx What I want to do is route all incoming trafic from my LAN that targets to my SIP server to it. I was trying adding this to SIP interface post-up ip route add [MySipServerIP] dev SIP src 192.168.1.0/24 table mgmt Another try: post-up ip route add [SIP ip] dev SIP src 192.168.1.0/24 table mgmt What is the right way to set this route?