There are three ways to access your voicemail using your phone's dial pad. You can: Press and hold 1. Dial *86. Dial your Xfinity Mobile phone number. NOTE: If it's your first time calling into voicemail, follow the recorded instructions to create a voicemail PIN. You’ll then record your name and set up a greeting.
Tripboba.com - Voicemail is a voice message that a caller will leave to your mailbox when you’re absent or busy with another conversation. The same goes if you’re trying to call someone else and they don’t answer. You can leave a voice message and they will receive the mail, allowing them to hear what you want to say.

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.
When you’re logged in, be sure that the mobile line displayed is the one with the iPhone voicemail password you want to change. Then follow these steps: Navigate the website starting with: Phone/Device -> Reset Voice Mail Pin -> Highlight your mobile number -> Submit; Once again, you’ll see “Password Incorrect – Enter Voicemail Password.”