Openwrt VOIP | MASQUERADE VPN Network Server to local

I have A Server and server have 2 Real IP IS ON USA i have a openwrt Router its Bangladesh and router have connected a VOIP DEVICE i want to send call THIS VOIP DEVICE now i have 2options 1) Install VPN on server and openwrt ROUTER and bridge network and set a static IP same serial on VOIP DEVICE useing MASQUERADE 2) make remote connections and USE 2nd IP on my voip device i need a solutions to pass call server to local VOIP device i cant use any PC or other device I have only a openwrt Router like TPlink Mr-3420 v2/v3/v5/ i want to run 32calls you can connect server to local a VPN and MASQUERADE connections or you can install any SIP PBX on local route like sofia-sip free-switch and register server to local if you want you can install a SIP Signal service on openwrt and pass call to voip device also you can bridge network and assin a server IP on local Voip device | its up to you| make sure it use low bandwidth if you have any another solutions You can share with me