I've read a few people have the same issue as myself.
I recently transferred over to VM but realised the voicemail or call forwarding wasn't working when someone called while I was already on a call.
I tried to go to my call forwarding settings on my phone but it comes up with network/SIM error.
I called the help desk and they apparently did something but nothing has changed
Any help would be appreciated as I'm ready to consider changing network if this can't be fixed.