I am happy to see contributions made here have been an inspiration for you.
So I must be benefiting from 4G Super Voice. With the modem placed in the window I am getting a consistent 30Mbs down and 11Mbps up which is fine for backup purposes (I've no PSTN phone so VoIP continuity is essential).
At those speeds, you are certainly not using Three's "SuperVoice" (VoLTE) 800 MHz band (Band 20) - you're going to be connected using the normal 1800 MHz band (Band 3). Band 20 is not notoriously slow, but in most cases the downstream is going to be slower than 3G in the single figures. You can use
LTEWatch to check your connection stats and force specific bands if you need to.
Plugging an analogue phone into the B310's phone socket also allows calls to be made from the mobile number, which is useful.
To my knowledge, B310 does not support VoLTE. But even still, Three only officially supports devices sold by them to use their VoLTE. Furthermore, VoLTE is only supported on Band 20 with Three. Though calls on 3G should not present any issues.
My advice is if you want to reliably make and receive voice calls via your router using the mobile number, it may be best to set the network mode to 3G only since auto induced some oddities for me on my B525 and 4G only calling will not work even if you forced Band 20, which is Three's only VoLTE-enabled band, since the router needs to support VoLTE and needs to be supported by SuperVoice.
I know my B525 does not support VoLTE (at least with the current firmware, since I believe there firmwares out there with this support). But by oddities related to auto mode, I think inbound calls would not switch the connection to 3G to receive a voice call (though, outbound was okay I think). You may or may not have the same issue.
If you have experienced something different, please post here because I would be intrigued