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I understand from your post that one of your contacts goes straight to voicemail even though they are not blocked. One step I recommend is to delete the contact and then add it back again. If the issue persists, check to see if any contacts are linked to the one contact that is going to voicemail.
Manage contacts - iPhone User Guide
To see if a contact is linked to another contact:Delete a contact. Go to the contact’s card, then tap Edit. Scroll down, then tap Delete Contact.
1. Open the contact, then tap Edit in the upper right corner.
2. Scroll down to the bottom of the page.
3. If any contacts are linked it will show up under Linked Contacts.
Tap each contact that is linked to the main contact and make sure none of them show as blocked.
Best Regards.