Configure Network for SIP trunk set up

First: Interface LAN was configured by installer itself, then it add mask /24 that must be removed.

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

Was modified to:

iface LAN inet static
    address 192.168.1.247
    netmask 255.255.255.0
    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.IP
    netmask 255.255.255.252

Then at /etc/rc.local must add two static routes

1. This route sends all trafic to LAN through LAN interface

   route add 192.168.1.0 gw 192.168.1.254
   

2. This route sends all traffic to my sip server through SIP interface

   route add [SIP SERVR IP] gw 172.xxx.xxx.GW
   

This all done the work.

Note:

172.xxx.xxx.IP is the Ip assigned by telecom provider

172.xxx.xxx.GW is the gateway assigned by telecom provider

[SIP SERVR IP] is the Sip server ip assigned by telecom provider.