All other major networks and some MVNO's have visual voicemail. For the iphone base, it's very easy as it's a one size fits all sort of thing. After being with Vodafone for 2 years, the idea of going...
New2Smarty, RCS is supported in GBR with at least com.google.android.apps.messaging's messages.android_20250908_01_RC00.phone.openbeta_dynamic (289312100) update on my Fairphone 5.
For everyone else, see https://community.smarty.co.uk/t5/chats-hacks/visual-voicemail/m-p/67/highlight/true#M16 for the official response to this feature request.