I use the LEX L11 on FirstNet and have no issues with Visual Voicemail but WiFi calling is another story as it does not work. I have not had a full time to investigate it further because I usually just Call Forward the phone anyways.
As of note I used the Sonim XP8 and really seemed to love it but then one day something happened with the ATT network. I woke up one morning and could no longer call forward at all. Long story short the Call Forwarding would not reach the LTE side of FirstNet and it would not work at all. I called FirstNet technical support, swapped SIMS, and even found the remaining XP8 in an ATT store in ALL of the Twin Cities. Still no luck and that is when I found that the L11 will not only work on FirstNet but on ATT, VZW, and to some degree on TMbl. Not to also mention it acts as a spkr/mic for my APX6000 and APX2000 I was sold.
I purchased mine through Magnum Electronics and the software for set-up is also free on MOL (just need an account). Once again only drawback no WiFi Calling. However, the PTT apps I use (Zello, DV Switch, and another PTT one) allows me to use the PTT button on the phone which is a HUGE plus.