To access voicemail from another line, call the phone number, let it ring till it goes to VM, then interrupt the greeting by pressing * and entering the PIN. You can also call 8056377243 > press * > enter 10 digit phone number > enter PIN to access your voicemail.
Website: https://shoretelcommunity.force.com/s/article/How-to-Manage-Your-Voicemail-Greetings-Using-a-Cisco-Phone

How to Record Voicemail Greeting on iPhone Tap the Phone app to open it. Tap the Voicemail tab. In the Voicemail screen, tap Greeting . In the Greeting screen, tap Custom. This is where you'll record your voicemail greeting and stop using the default... Tap Record and start speaking. When you're done recording the voicemail greeting, tap Stop . See More....
On the iPhone, go into voicemail and tap "Greeting". Record a few seconds of something and tap stop. Before you tap save, use iFuntastic or SFTP to navigate to /var/root/Library/Voicemail on your iPhone. You'll see a file called "Greeting. amr". This is the audio you just recorded. Delete that file and replace it with your custom message which should then be renamed to "Greeting.amr". Tap "Play" on the iPhone to verify your message works. If it plays, tap "Save" and your custom message will be uploaded to AT&T's servers.