Not able to transfer a voicemail greeting to iPhone

I understand your frustration however, they are the ones who control your voicemail. You will need to work with them.

Just FYI, I don't think you can record a voicemail that way. I've never heard of anyone sending an audio file to use as a greeting, normally you just record it on their servers

Contact your cellular provider on how to update your greeting.

Voice-mail is a carrier feature.

Then contact your carrier and ask them how to do it.