If you requetsed a phone number to be transfered to Public Mobile, this could mean that the phone calls are going to the carrier of the service that you are trying to leave. That would happened if that sim card has been removed and is in a powered up device.
If that's not the case, calls going to straight to voicemail can indicate a few things. Those include poor/spotty signal/coverage, a fluctuation in network conditions (capacity, or cell site "breathing" - a shrinkage of coverage area that can occur when many people are connected to a cell site), or a Public Mobile back-end routing issue.
On your end, this could be caused by a defective device, call management (blocking) settings, DND settings, the wrong network being selected or wrong network type, or pretty much anything that could cause the phone to not be connected to the network (included being turned off). I'm aware that you know that you phone isn't turned off; I'm just stating possible reason for calls going to voicemail that could happen to anyone.
