Voicemail lose

I currently have iPhone8 since last IOS update my voicemail has changed I have poor volume on my phone don’t hep situation but I had at a glance of old and new voicemails and in particular ones fromw my sister I don’t want to lose but it’s all gone. I can access VM onscreen button to press taking you through menu section but so far unable to retrieve saved voicemails my provider is o2. Is this because I have old phone or is this mandatory for all phones now. Anyone know what happening to our voicemail system please.