NAT UTM to XG

Basically you have one NAT and one Firewall Rule.

Use the Wizard to create the second rule as a NAT rule. It will generate this as you have it on UTM.

Then create a normal Firewall Rule for inbound to your WAN host as a Firewall rule. That should do the trick. 

Consult with your VOIP Provider about the actual needed scenario. Because this looks odd to me as well. Maybe you have the SIP Helper enabled on UTM? 

Hello,

I think you want to use the "Linkus" service of your Yeastar PBX-appliance.

Have a look here, you have to do a DNAT (port forwarding with Destination NAT) to your server, not the whole internal network!

https://help.yeastar.com/en/s-series/topic/port_forwarding.html

To define this kind of NAT-rule, I would recommend to use the "Server access assistant (DNAT)" wizard to define this.

You can find this here: