While I agree that this might be a practical and effective step... changing my number costs me a lot of money in terms of lost revenue and expenses, I have business cards to reprint and contacts (business and personal) that I'll need to update. Considering phone companies could simply block these calls if they so desired I don't see why I, the customer, need to absorb such a large cost just to keep my life sane and, after absorbing such a cost the robo calls might just find me again.
I receive more spam calls than spam emails and, being familiar with the underlying technical details of each system, that is absolutely ridiculous. Email was built in a time of pure optimism, there is no central switching authority for emails and there is no cost to sending an email other than jiggling some electrons - telephone calls are highly metered and monitored, they generally pass through several organizations that have been endowed with a high measure of trust to police and shepard them to their final destination. There is no reason other than the monetization of the system that these spam calls are allowed to exist.