Sure thing.
First of all I start with 19.07: I reflashed the router so to get the original behaviour.
config interface 'lan'
option type 'bridge'
option proto 'static'
option ipaddr '192.168.1.1'
option netmask '255.255.255.0'
option ifname 'eth0.1 eth0.5'
config switch
option name 'switch0'
option reset '1'
option enable_vlan '1'
config switch_vlan
option device 'switch0'
option vlan '1'
option vid '1'
option ports '6t 0 1 2'
config switch_vlan
option device 'switch0'
option vlan '2'
option vid '100'
option ports '6t 4t'
config switch_vlan
option device 'switch0'
option vlan '3'
option ports '6t 3t'
option vid '5'
I tried to ping from my PC (untagged vlan on LAN1, when I took the screenshot the cable was not attached yet) and the ATA box received it. On the router itself, I ran a tcpdump on br-lan interface and noticed that I could see the ICMP packets: this makes perfect sense, since the bridge is in charge of removing and adding the tag and so traffic has to flow over it.
Pinging another pc on an untagged port works perfectly and no traffic appear on br-lan; also this makes sense because, in this case, since we are in the default vlan1, all traffic goes through fabric and the bridge is not involved.
Now let's go to 21.02. Right now, the best way to have the voip device working is the following, that you don't approve of 
config device
option name 'br-lan'
option type 'bridge'
list ports 'lan1'
list ports 'lan2'
list ports 'lan3'
list ports 'lan4.5'
In this way, the voip box connects to the internet and works just fine. But when I try to do the same experiment as before, ping doesn't work and, funny thing, I don't see any traffic on br-lan.
Traffic doesn't seem to reach the bridge but is switched directly in fabric.
Using bridge vlan filtering causes a lot of problems, as described in other topics too:
MT7621 can not enable vlan_filtering on kernel 5.4 (where it is linked to the dual core nature of MT7621 and the fact that the fabric itself include the wan)
MT7621 bridged vlan with tagging not working properly Tagged host cannot ping untagged host (same device and situation from august 2020)
https://lore.kernel.org/netdev/E1jEB0y-0006iF-5g@rmk-PC.armlinux.org.uk/