Hey Erinview,
Welcome to Apple Support Communities.
If you haven't already, we suggest restarting the iPhone. Restarting has been known to resolve some software related issues. To restart, follow the steps below or in this article: Restart your iPhone.
1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
If restarting doesn't help, try adjusting the volume while the voicemail is playing. Check to see if you're able to hear when its on speakerphone.
Also, make sure your iPhone software is up-to-date. The latest version is iOS 17.0.3. It's best to keep your iPhone up-to-date since they include fixes and security updates. See steps below to back up and then update your iPhone: Update your iPhone or iPad.
1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see more than one software update option available, choose the one that you want to install.
5. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do.
Take care.