SIP trunk with dual IP. How will SIP invite work

I have a situation where I am using a dual IP - two IP on asterisk one IP is dedicated to SIP trunk server whereas the other one is connected to LAN.

The phones are registered to LAN IP: 192.168.X.X
Where is the SIP trunk is communicating via : 172.29.X.X to the SIP server.

The incoming call is working fine as SIP server is able to point to 172.29.X.X
But when I am sending out calls to the SIP server. My SIP invite contains 192.168.X.X as result I am getting SIP 503 service unavailable. Is there a way when sending SIP invite I can use 172.29.X.X.
The phones will always be register to 192.168.X.X

Asterisk should use the IP address associated with the interface with the best route to the destination, however, you might want to look into treating this as though it were a NAT situation.

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