Thanks for posting in the Apple Support Communities tryline,
We understand you're receiving continuous voicemail notifications. Do the notifications seems to correspond with actual voicemails? If so, we recommend reaching out to your wireless service provider. Since they manage voicemail for your line, they'll be in the best position to assist.
If the notifications don't correspond with new voicemail messages, try tapping Settings > Notifications > Phone, and toggle Allow Notifications off temporarily, then toggle it back on again.
Use notifications on your iPhone, iPad, and iPod touch
We hope this helps!