Hey lonesnark,
If you call to your voicemail either by dialing your number from your phone or using another device, are you able to hear the voicemails then? From Check your voicemail on iPhone - Apple Support
Check your messages when Visual Voicemail isn’t available
On another phone: Dial your own mobile number, press * or # (depending on your carrier) to bypass your greeting, then enter your voicemail password.
If you're not able to hear them using those steps, you'll want to reach out to your carrier as they control the voicemail feature. If you do hear it when using those steps, are you not hearing the voicemail when raising the phone to your ear, when using the speaker, or both? If you connect headphones, do you hear it then?
Keep us posted.
Take care!