Next to deleting all calls of the unsaved numbers via "Phone", "Recents, "Search" (I typed prefixes/area codes to select & delete them as efficiently as possible) you also have to delete all messages of the unsaved numbers (listed as "unknown").
The following steps are for a Samsung S9.
1. Open "Messages" App
2. Click "Search" 🔍 button
3. Type the phone number's digits until your "unknown" contact's messages pop up
4. Select all of them and then click "Delete" 🗑️
Optionally, you can do the following, to make sure the dialled number is deleted permanently (a "restore" could undelete the messages):
5. Back Up and overwrite your Phone and Message data via
➡️ "Accounts and Backup"
➡️ "Backup and Restore"
➡️ "Backup Data" on Samsung Account
6. Uncheck all Boxes except "Phone" & "Messages" and click "Back Up" button at the bottom of the screen.
Ipat144 your method won't delete the unsaved numbers. I just tried this and it doesn't work. Deleting the messages manually won't delete the unsaved numbers from the phone keypad system. Meaning when u dial only 3 digit of the following number, the whole number will pop up. How to prevent that number from popping up on the keypad was my issue. And it got solved by using SFM's method.
[Edit based on feedback]
There are two approaches to remove unsaved numbers, listed as "unknown", from the suggestions that pop up when dialling the phone's keypad.
1. Open "Phone"
2. Click "Recents"
3. Click "Search" and type the first digits of the number
4. Select & delete all logs of the unsaved contact listed in "Recents". This will delete ALL CALL AND All MESSAGE LOGS of the unassigned number, saved in your "PHONE" and the unknown number doesn't pop up via the keypad
If the unsaved number popping up in your keypad CANNOT be found via searching your phone's "Recents", the logs of the unsaved number are ONLY saved as "Conversations" in "MESSAGES" and can thus not be found via "PHONE".
To delete those logs, causing the keypad problem:
1. Open "Messages" App
2. Click "Search" button
3. Type the phone number's digits until your "unknown" contact's conversations are listed
4. Select all of them and then click "Delete"
OPTIONALLY, you can Back Up your Phone and Message data WITHOUT the unwanted unknown numbers, thus OVERWRITING the data WITH the unwanted unknown numbers. This prevents the numbers from ever popping up in your keypad due to a Restore that still contains the deleted logs.
1. Open "Settings"
2. In "Accounts and Backup" go to "Backup and Restore", then "Backup Data" on Samsung Account
3. Uncheck all Boxes except "Phone" & "Messages"
4. Click "Back Up" button at the bottom of the screen.