What can I do to avoid getting my phone number "less breacheable"?
Don't have a telephone number. Even if you never list it anywhere, a simple "brute force" attack (I.e. simply trying all possible combinations) will still work.
Apart from that, don't share your phone numbers, which would basically make the number useless.
There is no longer a good way to keep a phone number really private.
Block and move on.