Register your number(s) on the Do Not Call register and legitimate marketing companies and utilities will stop calling you after a few weeks.
Scammers will continue to call no matter what and can and will change their callerID regularly by spoofing.
The only memory being ‘chewed up’ by ‘blocked’ or unanswered calls is the few words of text you see. If you are worried about that clear your call history from time to time.
You can see how much memory your phone uses before and after by looking at the storage display in your settings. The difference likely will not be enough to change even the rightmost digit.
