When you tap Phone, iPhone shows the number of missed calls and unheard voicemail messages.

Tap Voicemail to see a list of your messages.

Tap the message. (If you’ve already heard the message, tap the message again to replay it.) Use
and
to pause and resume playback.
Once you listen to a message, it’s saved until your carrier automatically erases it.
Dial your own number or your carrier’s remote access number.