Retrieving voicemail from old icloud backup

I lost an important voicemail message when I got a new iphone. I backed it up to the iCloud on 25 April which was the last backup with the old phone. Since then it has obviously backed up the new phone whenever it is plugged in to charge.

How will I know if the voicemail was part of the backup? I just set up the phone to backup to iCloud when it was plugged in to charge. Was I supposed to do something else to backup voicemail messages?

If it is part of the icloud backup can someone tell me where the backups are stored in my icloud account, how to retrieve them and if I can select just the voicemails to retrieve or whether I have to restore to that full backup and lose everything added since that backup.

Please can someone help me with this. I am not techie and this voicemail is really important and I need it back if at all possible. Do I need to visit a Genius at an Apple shop to sort it out?