Thanks @urszula. I use Lebara Mobile (a MVNO that uses Vodafone’s infrastructure). Vodafone do support Flash SMS, but on my testing, when someone calls and leaves a voicemail, I receive receive 2 prominent indicators:
(1) in the centre of the screen: “1 missed call”.
(2) A count i.e. 1 appears on the phone app icon.
There is no indication of a voicemail at all.
Disappointing.
If any other users can test this voicemail indicator (that uses Flash SMS), I’d be grateful.