This goes a bit beyond enabling VoLTE from your SIM Card Network settings which if you haven't done you should enable before trying this... Of course you may not have the same results I had. I found it works best from Stock ROM no root needed I'm currently on 9.0.12 but tried it on 9.0.11 and Beta 5.
Like any mod you should make a backup now and proceed with caution. (By caution I mean don't deviate from instructions goofing around in any engineering mode is never wise)
You should try check your internet speed now (Google internet speed! And take note of it then run the same test to see if your able to improve at all.
Step 1: Optional step but for best results open dialer and type: *#*#4636#*#* From this Testing Menu select Phone Information. Once in phone info select LTE/UMTS auto (prl) for preferred Network type. Then scroll down a bit and make sure VoLTE is switched on. Exit the phone info screen.
Step 2: Open Dialer again and type: *#800#
A Warning Message to Engineering Mode will popup. Press Enter.
Step 3: Click on the first option Called OnePlusLogKit
Once in the LogKit scroll down until you see the menu called: FunctionSwitch now click and select it.
Step 4: There's 3 checkboxes of interest in this menu... VoLTE Switch, VoWifi Switch and VT Switch. I enable all three but some may have better luck not enabling VT Switch. After enabling each option it'll try and reboot your phone just say no until you have all you want enabled.
Now reboot and profit.
I can tell you without a doubt that if your using a stock OOS Rom and using TMobile as your provider that these changes will not eat up your battery any more than normal and altho depending on your location and towers speed availability this can and will increase your overall speed and your ability to maintain a solid connection in low signal areas! It forces carrier aggration meaning it makes the modem in your phone connect multiple times over varied frequencies to achieve the highest level of connection. Like your old 4g phone x 3 up to 6 put together. So your speeds can be 600% improved. Of course the phone is technically supposed to do this on its own which it can occasionally but only in absolutely optimal situations. So not when you really need it too. Lol. So altho it might not work on carriers without this tech or overseas if u live in North America and use TMobile this will drastically help your phone whether you instantly notice it or not trust me it does!
f you notice anything negative or battery draining or your phone becomes even more radioactive simply go back into the menu and disable the Functions. If this helped you at all throw me a like. Enjoy.