Did the network provider say HOW they expect it to be indicated? Because I checked this on a traditional smartphone, here in POLAND- A Samsung S20 & the only way this phone gets info about voice messages is by the network provider PLAY sending an SMS.
I have to admit, I’m not entirely clear as to what to pass on to the team. HOW does your network (Lebara) notify users of a VM when they have, lets say an iPhone or an traditiona Android smartphone.
Some operators provide SMS when someone leaves a voicemail
Others doesn’t do that- I understand yours does not.
Others can use Class 0 message (I never saw that) which is a popup (But Mudita Kompakt does not have a NOTIFCATIONS TRAY)
Voicemail notifications probably need to be enabled or it is enabled by default depending on operator and tariff used.