Here’s the design flaw: Your iPhone will show a badge or alert when a blocked voicemail arrives. The red badge on the Phone app only counts messages in your main inbox. You could have 20 blocked voicemails sitting for months, and the phone will never notify you. They simply accumulate in silence.
To access blocked voicemails on an iPhone, follow these steps:
: Unlike voicemails, blocked text messages are never stored and cannot be retrieved. Blocked number leaving voicemails - Apple Support Community
While blocking a contact prevents their calls from ringing your device, Apple still allows them to leave messages that are tucked away in this hidden section to keep your main inbox clean.