Hello Sophie,
You have Call-Forward Immediate to Voicemail (DnD) set on the 5002 extension (*20*). Turn it off with *23* or *20*.
Group ring will ignore Call Forwards on individual extensions (it takes the outcome path instead). Direct dial will not.
I don't know why your audio stream is failing, there isn't enough debug info to tell. Perhaps you have added/changed something in the voicemail header or you aren't allowing GSM recoding
Kind Regards
Selintra