My ISP runs WAN link carrying Internet and Voice Vlans. Voice vlan should be switched and Internet vlan should be NATed. I havent tried the setup yet, but lets say the wan port on mx is set to tag for internet vlan, and i configured a local vlan for voice traffic. Would voice vlan be switched from WAN port to LAN port?
You would need to connect the handoff from your ISP into a switch, then, you can switch the voice VLAN and create an access port for the WAN, then connect your MX internet port to that. The MX will allow a VLAN for the WAN interface, but will not pass a second VLAN through it.