Slim can't do VoLTE, no AOSP I'm aware of can. You need NF1, stock. Note that you must update the modem and probably the bootloader as well. You can mod it, like Tweaked or the various other custom ROMs. And/Or Xposed. But you have to be on a TMO firmware to use VoLTE. Just how it is. And NYC should be fully VoLTE enabled as they are an official site.
I am in an unofficial area (Salt Lake) and it works fine on Tweaked. I don't really care about it, as I rarely talk on the phone, but I can see how people would like it. The big win as far as I'm concerned is that it opens up the possibility to re-purpose more spectrum for wideband LTE.
As for root/recovery, that's easy. You don't keep them with the Odin flash, but just boot back to download mode and flash TWRP from Odin and CF-AutoRoot from Odin or a custom ROM from TWRP. There's root. Now restore your apps via Titanium or Play Store, and you're done.