The most effective way is to look up their LOCAL BACK HOME District Office Phone Number. Call one near you and don’t block your number. Then they know it is someone in their district calling. They know they are talking to a voter inside their district which carries far more weight. Before C-19 you could also drive to a local district office and tell an aide in person.
Unfortunately, my area code is from CA, and I live in TN now. In fact, most of my friends here got their cell numbers in states where they lived previously. It almost would be worth it to get a land line for these basics.