By the way: I haven't actually used facebook since 2012...
The random "How are you are you single wanna talk about sex with me?!!???" messages are bad enough. I certainly don't want those as phone calls.
Those on the list that are actual Friends might have my phone number. Might not, though, since there is no point (I moved countries). In any case, they know not to freaking call me unless it is important or you have reason or you want me to answer my door.
That's only true for you if you choose it to be true for you.
Not all of these groups would have my phone number, but facebook isn't exactly a telephone service at its core.
I could include folks I've been to school with, folks I met in language class, a few folks I've worked with, and so on. I don't want all of these folks to call me, though, and I*d rather some didn't have my number. I also don't fully dislike these folks.
More seriously, though, it would keep my social circle smaller. I met my spouse online around 10 years ago, and this sort of caution you ask about would mean I wouldn't have my life.