No need for a box on Vodafone, you can use wifi-calling instead of 3G/4G
EE did this first in 2016, with iPhone 5c and later handsets, and a collection of Androids with EE firmware, and the service does both SMS and voice.
Vodafone were late 2016 and only support iPhone 6s and later, and a very limited set of Androids. They only support voice, no SMS.
Three launched this very recently and are the same as EE.
O2 don't yet support.
This of course is different to O2 and Three's apps, or Vodafone's SureSignal 'box' (femtocell) offering that plugs into a power socket and ethernet cable to your router and provides a 3G signal.
plusnet unlimited fibre
80/20 since 2 Jun 14 - Sync as of 5th Feb 17: 57,821/9,173 kbps with G.INP
18 years of UK broadband since 1999 ntl:cable modem trial -Router: Asus RT-AC68U with HG612 - BQM