MX doesn't support voice VLAN, so I'm not sure if it would work as expected.
Voice VLAN Overview
The voice VLAN feature permits the switch ports to carry voice traffic with Layer 3 IP precedence and Layer 2 class of service (CoS) values from an IP phone. Based on IEEE 802.1p CoS, the switch supports quality of service (QoS) which uses classification and scheduling to send network traffic from the switch. You can configure the Cisco IP phone to forward traffic with an IEEE 802.1p priority, and configure the switch to trust or override the traffic priority assigned by an IP phone.
You can configure the switch port, which is connected with an IP phone, to use one VLAN for voice traffic and another VLAN for data traffic from a device attached to the access port of the IP phone. You can configure access ports on the switch to send Cisco Discovery Protocol (CDP) packets in order to instruct an attached IP phone to send voice traffic to the switch by any of these methods:
In the voice VLAN tagged with a Layer 2 CoS priority value
In the access VLAN tagged with a Layer 2 CoS priority value
In the access VLAN, untagged (no Layer 2 CoS priority value)
The switch can process data traffic which comes from the device attached to the access port on the IP phone. You can configure the switch ports which send CDP packets that instruct the attached IP phone to configure the mode (trusted or untrusted mode) for the access port on the phone.
In trusted mode, the access port on the IP phone passes the traffic from the PC without any change. In untrusted mode, the access port on the IP phone receives all traffic in IEEE 802.1Q frames which contain a configured Layer 2 CoS value. The default Layer 2 CoS value is 0. Untrusted mode is the default.
Thanks for the reply. We don't have a switch involved in this setup and was hoping that daisy chaining a phone/PC off a single LAN port might have been possible instead of assigning an individual LAN port to both devices.