If the parent is aware that your 'app block scam calls' does that make them more likely to believe a scam caller, if one get's through your screening?
Whereas if you educate the parent about the persistence and ever increasing complexity of scams, and they experience the calls and are competent to dismiss them, you make them more aware and more resilient in future - Scam awareness training of sorts.
I’ve used RoboKiller before for myself. It did a fantastic job at nuking most every spam call. Maybe that would work for you?