Critique my network design - PBX / VoIP

Hi Guys,

We have had a new VoIP solution deployed in my company, I'm Solo responsible for the network within the org, I ran these designs by the VoIP guys, who were happy with everything but I am just looking for some more opinions.

https://imgur.com/a/dCW33JN - here is the diagram

Everything is working fine, apart from a QOS issue where the calls loose quality if I saturate the internet uplinks

Just to explain the setup in order we have:

1. Leased line/ Backup FTTC terminations - this is where the internet uplink comes into the company. The leased line is 100/100 and the FTTC is 60/40 (Down/Up)
2. Next up is the Juniper router, this is managed by the ISP. We have a total of external 5 IP addresses on this network. This is the gateway for our Firewall and PBX
3. After the ISP router we have our Sophos Firewall. here is where we define all our internal networks and VLANS. For this diagram only the 'Main LAN' and 'IP Phone' VLAN is relevant, so i have redacted the others. Our internal networks use this firewall as their default gateway, the firewall will then NAT and send the traffic towards it's gateway (juniper).

Also on the firewall I have a L2 bridge which allows me to connect the WAN port of the PBX to the outside world, withthe safety of the firewall. - No NAT

1. the PBX has 2 network ports. One for WAN, which requires an external IP and one for LAN which has an internal IP onthe 'IP Phone' VLAN.

2. Finally we have the internal networks. Here is where our switches and end devices sit

Now you've seen that i have a question.. If I wanted QOS on the PBX to guarantee bandwidth will this have to be setup on the ISP Juniper since this is the gateway for the PBX? it only passes the firewall using a L2 bridge, so i cant configure it here?

INb4: obviously if money was no object i would have a switch between the router and existing firewall, then i would have a 2nd firewall for the 'External' networks, which contains the PBX