[VoiceOps] Enterprise customer desiring NAT for their SIP

We have an enterprise customer that wants to trial our SIP trunking service. They're using Cisco Call Manager v8.5 and I tried to get them to use a public IP or a private VLAN to our network, but he insists that they want to keep their CCM behind their NATing firewall. Our softswitch has an integrated SBC, so our side is covered, but I'm afraid that they may run into some issues that are related to NAT but that we'll be held responsible. Should I continue to push back on this customer, or let them go down this road? Regards, Frank