Use an app like
BlackList. You can use wild cards, so 0* will block any number starting with 0. And if you learn a little about RegEx (Regular Expressions - there are tutorials all over the web) you can block all 800, 855, 866, etc., numbers in one line. Also all calls with blocked numbers, numbers not in your contacts, etc. (And even if you block all 800 numbers, you can put a single 800 number, or a regex 800 in an exception list, so that company can reach you.)
Oh, and it's free, except for the exception list - that requires the paid version. And blocking texts from Android 4.4 up requires Xposed Framework (free also).
