I can report the HTC One works well with the AT&T Microcell (which only supports 3g) that is in my home. Currently AT&T doesn't offer VoLTE, and voice is only carried over 3g or less.
Initially I was worried the Microcell might wrestle away the 4G LTE connection I'm able to get in my home, but this isn't the case. The phone (network?) is smart enough to connect to both the 4G macro-network for data & the 3G Microcell I have at the house for voice calls.
This lockscreen pic shows the One connected to the Microcell- and represented by the bars. The 4G icon shows it's also connected to the 4G LTE network- plus the wifi network I have at the house. Inbound and outbound voice calls are routed to the microcell. Data uses wifi, or LTE if I turn wifi off.
AT&T's Microcell used to be an expensive piece of crap. But with the firmware updates a year ago, it's now just expensive. It actually does work pretty well at this point, and so far is playing nicely with the HTC One. At $199 though, plus the extra fee to not eat your minutes, it's a giant rip-off. I was able to purchase mine 2+ years ago when they first became available in my area @ $99.