@nzbsgfan has confirmed that 111 over VoLTE is currently only rolled out on the RCG towers. On native Vodafone sites, call is still passed down to 3G. So if you have no 3G coverage - e.g. phone is locked to 4G or in an area with 4G only coverage (e.g. a 4G only Cel-Fi) then 111 call will fail.
This worried me as we have phone over RBI on a native Vodafone site, and I set the modem locked to 4G. So today I did a test call to 111. It worked with no problem.
So not only the RCG towers?
To be clear - are you talking about a fixed wired phone plugged into your RBI modem's TEL port?
If so, I'm thinking coffeebaron may have a point - during a voice call your modem may be CSFB to 3G - I think the 4G setting is 4G prefferred - not locked.