Let says that You're really wanting someone on Your Facebook friends to be Your girl friend or boyfriend, or just to take her/him to a date, that You're interested with her/him by finding their phone number on their contact profile, but You can't find them any. Anyway, since Facebook changed the majority of the users into Timeline profile appearance, some of You might be asked "Where is the phone number stored on Facebook?", or "Where do I find phone number of my friends on Facebook?", or "How do I find phone number of Facebook users?", etc. If You're about going to find the phone number of Your Facebook friends, unfortunately You can't do that, it's because Facebook will only appear the phone number of users on their Timeline (profile) as a default setting, Unless Your friends make their phone number visible to public, or friends. You can choose who can see Your mobile number.

Here's what I got from Facebook Help Center:

Q: If I use my phone number to confirm my identity, will it appear in my profile (timeline)?

A: Your mobile number will only appear on your profile (timeline) if you want it to. you can always control who can see your number.

So, Why Facebook such a hide the phone number of users visible from their friends, or public?, I think the answer is because the privacy issue of the users. Which You might have heard the rumors, or hoax that Facebook published the phone number of the users, but Facebook has claimed posted via status updates it was not true. 

Then You might asked Yourself, "Is there any way for me to find the phone number of my friends when I can't see them on their friends profile?, of course there's always the possiblity to do that, as long as You're not give up. My suggest is why don't You ask bravely her/his phone number wisely and politely to them via her/his inbox?!. It's worthy to try right? fight for Your loveanyway :D