I don't know if it's a new trend or something. But most of these guys that insist on getting your phone number are the ones that never call. Instead once they get the number, they run to WhatsApp to add you and the next thing you get is a WhatsApp chat that says;
"Hi baby, good to have you here". Like what the hell?

Well, I have a trick for that. I have a number that isn't registered on WhatsApp. So if they insist like their lives depend on the number, I just give it to them, knowing fully well how it works. Most times, I usually get a call after two days from them;
" Hello, it's John, the guy you gave your number few days ago. It seems you are not on WhatsApp on this number. Please can I have your WhatsApp number "?.
Can you imagine? Give you my WhatsApp number so you'll kill my battery with endless chats.
Some of these chats can be so annoying and IQ reducing. You will see chats like:
" Sup", "Howdy"?, " Wadup"? "What are you up to"? " fine geh, talk to me nah"... The list is just annoyingly endless.
Some even add you and immediately start doing the WhatsApp call. And then you get a message; " why are not picking my calls or are you busy"?. If I don't take your WhatsApp calls, why not call the normal way if the call is that important.
It gets more annoying when they start sending you copied messages that require you to send to all your contacts and blah blah blah...

Read more here
http://www.peacedike.com/2016/08/whatsapp-guys.html?m=1