How do I reset the voicemail number on galaxy J5 after it has been set?

On my work mobile the voicemail records my greeting but it's not working when people call it. It's a Samsung J5 2017. I've tried multiple things to find a setting to change it but with no joy. My office bt line is being redirected to it but I'm not sure if that affects anything. The network provider is BT mobile.  When I call voicemail it says there are no messages, as if it is working. And when I go to personal greeting it plays me the recording and says it is set.

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