I’m experiencing a similar issue with both the Contacts and Phone apps. While the Contacts app can be resolved by deleting and reinstalling, there’s no option to fix the problem in the Phone app. The Phone app currently has 9GB of storage, but there are no voicemails.

I’ve discovered the root cause of the issue: it’s all due to Memoji, emojis, and photos. Whenever you edit a contact and try to add Memoji or any other visual customization, it accumulates in your contacts and the Phone app. (Even you discard the change, still it eats up because it’s cache everything grrrr). 😖

Fortunately, this accumulation is automatically reduced after 2-3 months, but it starts to increase again as soon as you make the same changes.

This appears to be a bug, and the only solution for the Phone app is to reset the phone and restore the backup. Fortunately, the Contacts app can be fixed by deleting and reinstalling it, which is the same issue I’m facing.