Voip - NAT & Invite different?

Question

We have an external network where a voip server sits on (192.168.5.0/24) 
 We have clients on an internal network 10.1.1.0/24 
 Client 1 10.1.1.1 NATs to 192.168.5.1 and so on with the other clients so that all clients appear as 192.168.5.x to the voip server on 192.168.5.250 
 Problem is, the voip server see's the invite coming from the original ip address and not the nat'd address which results in it being rejected. 
 Now turning the Voip helper on gets us a little bit nearer but according to Sophos, you shouldn't use standard NAT eg SNAT/DNAT with this. 
 So my question is..... how do I present the the client 10.1.1.1 as 192.169.5.1 without having a NAT in place?

Argo · Answer

Hi Louis-M, 
 
 I use VoIP through a UTM without any issues at all. 
 
 I always use MASQ (unless there is a specific reason the use both SNAT and DNAT rules) when going from one network (or device) to an external server. 
 Currently I have used both scenarios (NAT & MASQ separately) with different customers. 
 I also place the client/network and sever in the appropriate section within, Network Protection --> VoIP --> Sip (I am presuming you are using SIP) 
 I only use the VoIP Helper when I know the exact bandwidth that is supplied to the location, otherwise I have always found it gets in the way of other services.