The default GOS Phone app has an option to block unknown calls.
Open the phone app ->click on 3dots ->Settings ->Blocked numbers -> enable Block calls from unidentified callers.
In the same place itself there is an option to manually add numbers to block calls and sms, im not sure whether the first option itself will block the sms from unknown numbers.
The Default sms app donot generate previews for links you receive, also just keep in mind not to click on unknown links.
Lockdown mode is for people with high treat model, if you have that i think should not consider using the traditional calls and sms. Just disable those apps.