I've tried using it. It is inconsistent and doesn't work the same.

If someone wants to share a phone number, they have to share it on the particular person's wall.
These are 3 examples on how phone numbers work on the site.
Lets take this post for example.
1) Say you want to send me your phone number. You have to click on my name or picture then write me a message with your number. Or vice versa. Nobody else will see it.
2) Now, if you just wrote your number under this thread only, ONLY YOU could see the number, nobody else could because you'd be the poster but since you started the wall you're the wall owner too. So only seen by poster and wall owner, and in this example you're both.
3) Now, let's say someone else started the thread. I'll throw out @A myHIVteam Member for example. Say happydays tagged you in a post on his wall. Then we started talking. You then tag my name and you give me your number. I will not see your number. Only happydays and you will see it, for happydays would be the wall owner by starting the thread, you'd see it by being the poster and it would be hidden for me not to see even though you tagged me.

I did not know that was available! Nice to learn.

I prefer the website, myself.

I use both. The website has things the app dont. The app has things the website dont.