Mikrotik SIP trunk routing

I have a Mikrotik RouterOS v6.36 with my network as follows
(I am a newbie to networking in general and have been able to get stuff working half way. I just don't know what to look for.)

• Router:
  • Ether01: ISP Connection(Internet/WAN)
  • Ether02: SIP Trunk(SIP Trunk – No internet)
  • Ether03: Null
  • Ether04: LAN

I have a PBX server on the LocalLan that needs to connect out via Ether01 to authenticate licenses etc. This works fine with no problem.

I now need to connect Ether02 which is a SIP trunk from our ISP to our PBX server.

I am able to get the Router to mark routes and send them out via Ether02 but I am not able to get that data to respond back through Ether02

Please could somebody give me guidance and let me know what data is required to help troubleshoot this.