VPN DECT SETS

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Good Evening all, we are currently deploying a OXO with Rainbow, fixed users and DECT on site (ip) and remote vpn sets using FORTIGATE firewall.

The VPN handsets work fine but the client is asking can we do a VPN cordless set ? I have tried to configure a YEALINK set, using OPEN SIP. I get it registered (without the need to VPN just using the PUBLIC IP and NAT). The set 'works' in principle ie it can dial out and it externally and internally but no audio is passed through. I have opened up ports on firewall on both sides to no avail. There is a VPN section in the YEALINK but no template to assist in setting up. Does anyone elses have any ideas or an alternative cordless remote phone ? Note there is NO vpn tunnel between HQ and remote sites hence using the FORTIGATE. Thank you in advance