The OP didn't exhaust their solutions. They found a simple solution which works (for them), and shares it. Once the solution gets widely adopted, it will also get more attack surface. In the meantime, the solution works well because the risk (getting spam calls once more) is worth the small investment. Its essentially a no-brainer to implement this, if you suffer from this issue (I don't, I'm eagerly waiting for a fun phone call with Miclosoft customer support despite owning no such products).