On dating websites, I see a huge red flag when a woman asks to chat on whatsapp because it means she wants to know the phone number of her interlocutor.
Nowadays, we don’t know what a woman can do with our phone number. She may use it to send you spams. She may resell your phone number to someone. She may then try to scam you using the phone number using a different nickname. This person can also send text messages to your phone number and call you. Also, since they know your phone number, scammers may make you believe they know you.
Conversely, you don’t take such risk when you give away your Instagram or your telegram ID. For your information, you can easily change your telegram ID. If somebody knows these ID’s, you can block all incoming messages of people you don’t know.
In other words, don’t give away your phone number to someone you don’t know.