Our RBI not only switches to 3G for voice but also switches towers (to a more remote site) and as a result I loose internet service for duration of call. Vodafone not very helpful on problem.
(they should give me a second RBI service for phone only until they solve network problem e.g lock my modem to a particular tower or provide VoLTE)
Have set B525 modem to "4G Only" to "fix" problem (and use mobile phones for voice)
Don't want to go data only on RBI as I want to keep my landline number for future